In general, the indefinite article ("a" or "an") in a condition on a card means "at least one". Examples of this include—
Worship ("If you control a creature..." means "If you control at least one creature..."),
bloodthirst N ("If an opponent was dealt damage..." means "If at least one opponent was dealt damage..."; see also this thread) (C.R. 702.54a), and
Myojin of Cleansing Fire ("...as long as it has a divinity counter..." means "...as long as it has at least one divinity counter...").
In a condition, where "at least one" is not meant, alternative wordings would generally be used, as found in Deadly Wanderings and Tidal Influence (in its Oracle text [C.R. 108.1]), for example.
As you correctly point out, that effect modifies the rules of the game (C.R. 611.2c). However, a ruling on The Chain Veil indicates that the effect is cumulative (compare The Chain Veil with Panharmonicon; see also C.R. 101.1 because the effect overrides C.R. 606.3 in part).
To exile an object means to "move it to the exile zone from wherever it is" (C.R. 701.10a); thus, the object will necessarily have left wherever it was. And Reality Acid's last ability will trigger whenever it leaves the battlefield, even if it goes to exile and even if it reenters the battlefield immediately afterwards (as can happen with Brago's last ability) (review C.R. 603.6c).
You can't move it to the Command Zone if it would enter the battlefield. It will just enter the battlefield.
Also worth noting is that the effect putting it into the Command Zone is a choice made by that card's owner. If you don't want to put it into the Command Zone you don't have to.
That's what I thought as well. Apparently MTGO disagrees because when I O-Ringed his Journey the Commander immediately moved into my Command Zone for some odd reason when we had both expected it to hit the battlefield.
If the commander moved from exile to the command zone rather than from exile to the battlefield, then that is a bug in Magic Online (as of the rule update with Core Set 2021, a commander's owner can choose to move their commander to the command zone only if it would otherwise go to a player's hand or library, or as a state-based action if the commander is in exile or in the graveyard soon after just having moved there [C.R. 903.9a-b]). Referring to comment 1, note that a player doesn't "cast" Oblivion Ring or Journey to Nowhere "on" another permanent (with limited exceptions, no permanent spell can have targets [C.R. 115.1]), but rather a player targets an appropriate permanent when its enters-the-battlefield ability is put on the stack (C.R. 117.3b, 117.5, 603.3d, 601.2c); thus in the scenario in comment 1, Oblivion Ring or Journey to Nowhere will already have been on the battlefield by the time Journey to Nowhere or the commander, respectively, is exiled.
EDIT (Nov. 21, 2019): Correctness edit. Some rules were renumbered with Core Set 2020.
EDIT (Jul. 2, 2020): Edited to conform to rule update with Core Set 2021; the rules changed so that a commander that's exiled doesn't necessarily go to the command zone immediately.
EDIT (Aug. 14, 2020): Further edited to conform to recent rule changes.
EDIT (Jul. 26, 2021): Edited.
In general, Bloodrage Vampire would enter with at most one +1/+1 counter on it, no matter how many opponents were dealt damage this turn.
There are two forms of bloodthirst (C.R. 702.53). "Bloodthirst" followed by a number, which appears on all cards with bloodthirst except Petrified Wood-Kin, means the permanent enters the battlefield with the stated number of +1/+1 counters on it, as long as "an opponent", that is, at least one opponent, "was dealt damage this turn" (C.R. 702.53a), even if more than one opponent was dealt damage this way or more than 1 damage was dealt this way. Note also that neither form of bloodthirst is a triggered ability, since its expanded rules text doesn't start with "when", "whenever", or "at" (C.R. 702.53a-b, 603.1).
Activated abilities of the form "Prevent the next...", like most other activated abilities, can be activated any time their controller has priority, whether or not damage is about to be dealt (because a spell or ability that would cause damage is on the stack, for example) (C.R. 116.1c, 615.3). What should be kept in mind is that for such an ability to prevent damage meeting its criteria, that ability must resolve (and the prevention effect be created and exist) before such damage would be dealt (C.R. 615.4, 609.1). Moreover, prevention effects can't undo damage that's already been dealt (C.R. 615.4).
In a given combat damage step, combat damage is assigned and dealt before anything else happens in that step (C.R. 510.1, 510.2); as a result, in general, a prevention effect like Argivian Blacksmith's that could prevent that combat damage must be in effect before that combat damage step (for example, by activating Argivian Blacksmith's ability during the declare blockers step [C.R. 509.4] or earlier in the turn [C.R. 500.1, 506.1]).
In this scenario, the original Master Biomancer's power, but not the power of the tokens entering, is relevant in determining how many +1/+1 counters each token enters the battlefield with. Note that each token's effect doesn't modify that token's power, and that each effect doesn't "already exist" (under C.R. 614.12) before all those tokens enter the battlefield at once.
With the understanding that no Magic card (other than "silver-bordered" cards) has an effect like that of Grusilda's abilities:
The comprehensive rules don't include any rules on "combined" permanents or what happens when a permanent enters the battlefield "combined". The answer ultimately depends on what rules the players in the game adopt for "combined" permanents (perhaps with inspiration from the reminder text on Grusilda). Flipped is a status under C.R. 110.5, and C.R. 709.2 says that "(o)nce a permanent is flipped, its normal name, text box, type line, power, and toughness don't apply", but rather their alternative versions. See also the example in C.R. 110.5c, which clarifies what happens when a permanent that isn't normally a flip card is flipped.
In general, in a game allowing "silver-bordered" cards such as those in Unglued, Unhinged, and Unstable, the players in the game can agree on modifications to the comprehensive rules ("house rules") to accommodate situations, such as this one, that the comprehensive rules neither regulate nor answer and that are unique to such cards (see also C.R. 100.7). Although Mark Rosewater issues "rulings" on how certain game situations unique to such cards play out, such advice is no more or less valid than the "house rules" agreed to by the players (that is, such players can agree whether to adopt such "rulings" or not).
EDIT (Jan. 16): Correctness edit in view of changes to the Magic Tournament Rules with Rivals of Ixalan.
EDIT (Mar. 4): Edit last sentence of second paragraph; apparently the rules do clarify the situation with permanents not normally flip cards.
EDIT (Jan. 2, 2019): Edited.
EDIT (Jul. 9, 2019): Some rules were renumbered with Core Set 2020.
In this case, Omen Machine overrides Shared Fate's replacement effect. Since players can't draw cards, Shared Fate can't replace that with something else. In general, if something is impossible, it can't be replaced (C.R. 614.7, 101.2, 101.3; see also C.R. 614.17c). See also this thread.
I have a question about necropontence. Will I still draw cards from my exile with Necropotnece if they were put in there by a different source. Say I exile most of my library with Demonic Consultation to find necropotence. Then next turn I play necropontence, at my endstep will I draw those cards from exile?
First of all, note that Necropotence's last ability doesn't involve "drawing" any cards (review C.R. 120.1; see also C.R. 120.5). Also, with Necropotence, the phrase "that card" on Necropotence's last ability refers only to the card that you exiled with that ability, not to cards in exile that were put there by any other means (such as by Demonic Consultation).
Yes. Remember that a spell, such as Overrun, will resolve only if all players pass in a row while Overrun is on top of the stack (C.R. 116.4). And at the end of this process, Overrun is still on the stack, so Player B can still target it with Counterspell (C.R. 114.1a, 110.1) -- that player is not required to have Counterspell in his or her hand right after Overrun is cast this way.
Sources of damage include lands (C.R. 119.1, 119.2; under C.R. 109.1, 109.2, and 110.4, objects include lands), and the appropriate damage is prevented by Urza's Armor whether that damage is dealt while a mana ability is resolving or otherwise. For Rune of Protection: Lands, which is what you probably have in mind, a land can be chosen as a source even if that land is not capable of dealing damage and even if that land is not about to deal damage (C.R. 609.7a).
In effect, you controlling Phyrexian Unlife means, among other things, that the state-based action described in C.R. 704.5a (losing the game for having 0 or less life) doesn't apply to you (C.R. 101.1). However, C.R. 704.5c (losing for having ten or more poison counters) still applies to all players, since no card mentioned in your comment overrides that rule; but Melira or Solemnity will keep you from getting further poison counters (C.R. 101.2, 101.3).
In this scenario, Leeches still deals as much damage as poison counters you lose due to that spell (which can be greater than 0 if you had poison counters before Melira or Solemnity entered the battlefield) (note that under C.R. 120.4, there are several parts to dealing damage), but even though Phyrexian Unlife makes that damage be in the form of poison counters (C.R. 120.3b, 609.4), you don't get poison counters this way because of Melira or Solemnity (C.R. 101.2, 101.3).
EDIT (Dec. 30, 2019): Edited, including because some rules were renumbered with Core Set 2020.
EDIT (Sep. 30): Correctness edit in view of recent rule changes.
Languish doesn't destroy any creatures, and for all three creatures you mention, Languish will bring their toughness to 0 or less, sending them to the graveyard as a state-based action (C.R. 704.5f, 117.3b, 117.5) (which is not the same as destroying them, however, so that totem armor, regeneration, and indestructible will not affect that event [C.R. 701.7b; review C.R. 702.88, 701.14, and 702.12, respectively]). However, if Languish doesn't bring a creature's toughness to 0 or less, but the creature has damage that equals or exceeds its new toughness, that creature would be destroyed as a state-based action (C.R. 704.5g, 117.3b, 117.5), so that totem armor, regeneration, and indestructible could change the situation (review C.R. 702.88, 701.14, and 702.12, respectively).
Moreover, Languish doesn't target anything ("all creatures" doesn't indicate a target) (C.R. 115.1a, 115.10a), so Slippery Bogle can be affected by Languish despite its hexproof ability (see its Oracle text [C.R. 108.1]) (review C.R. 702.11b).
EDIT (May 3, 2019): Some rules were renumbered with Dominaria.
EDIT (Dec. 2, 2019): Some rules were renumbered with Core Set 2020.
EDIT (Jan. 17, 2020): Correctness edit.
EDIT (Feb. 19, 2020): Add rule citation.
In a condition, where "at least one" is not meant, alternative wordings would generally be used, as found in Deadly Wanderings and Tidal Influence (in its Oracle text [C.R. 108.1]), for example.
However, the foregoing generally doesn't apply—
Only few Magic cards have the phrase "only one", and they include Meddle, Quicksilver Dragon, and the Unglued card Once More with Feeling in its Oracle text (C.R. 108.1).
EDIT (Jan. 2): Added paragraph.
EDIT (Jan. 16): Correctness edit in view of changes to the Magic Tournament Rules with Rivals of Ixalan.
EDIT (Mar. 1): Added paragraph.
EDIT (Feb. 21; Mar. 10): Clarification.
EDIT (Sep. 11): Correctness edit.
EDIT (Mar. 27, 2021): Edited generally.
EDIT (Sep. 1, 2022): Correctness edit. One rule was renumbered in the meantime.
EDIT (Nov. 21, 2019): Correctness edit. Some rules were renumbered with Core Set 2020.
EDIT (Jul. 2, 2020): Edited to conform to rule update with Core Set 2021; the rules changed so that a commander that's exiled doesn't necessarily go to the command zone immediately.
EDIT (Aug. 14, 2020): Further edited to conform to recent rule changes.
EDIT (Jul. 26, 2021): Edited.
This is set out in C.R. 109.2.
There are two forms of bloodthirst (C.R. 702.53). "Bloodthirst" followed by a number, which appears on all cards with bloodthirst except Petrified Wood-Kin, means the permanent enters the battlefield with the stated number of +1/+1 counters on it, as long as "an opponent", that is, at least one opponent, "was dealt damage this turn" (C.R. 702.53a), even if more than one opponent was dealt damage this way or more than 1 damage was dealt this way. Note also that neither form of bloodthirst is a triggered ability, since its expanded rules text doesn't start with "when", "whenever", or "at" (C.R. 702.53a-b, 603.1).
EDIT (Jan. 30, 2018; Jun. 6, 2018): Correctness edit.
In a given combat damage step, combat damage is assigned and dealt before anything else happens in that step (C.R. 510.1, 510.2); as a result, in general, a prevention effect like Argivian Blacksmith's that could prevent that combat damage must be in effect before that combat damage step (for example, by activating Argivian Blacksmith's ability during the declare blockers step [C.R. 509.4] or earlier in the turn [C.R. 500.1, 506.1]).
EDIT (Mar. 25): Correctness edit.
EDIT (Mar. 30): Correct rule citation.
EDIT (May 14, 2019): Correctness edit.
EDIT (Feb, 20): Stricken out, then quickly unstruck out; correctness edit.
The comprehensive rules don't include any rules on "combined" permanents or what happens when a permanent enters the battlefield "combined". The answer ultimately depends on what rules the players in the game adopt for "combined" permanents (perhaps with inspiration from the reminder text on Grusilda). Flipped is a status under C.R. 110.5, and C.R. 709.2 says that "(o)nce a permanent is flipped, its normal name, text box, type line, power, and toughness don't apply", but rather their alternative versions. See also the example in C.R. 110.5c, which clarifies what happens when a permanent that isn't normally a flip card is flipped.
In general, in a game allowing "silver-bordered" cards such as those in Unglued, Unhinged, and Unstable, the players in the game can agree on modifications to the comprehensive rules ("house rules") to accommodate situations, such as this one, that the comprehensive rules neither regulate nor answer and that are unique to such cards (see also C.R. 100.7). Although Mark Rosewater issues "rulings" on how certain game situations unique to such cards play out, such advice is no more or less valid than the "house rules" agreed to by the players (that is, such players can agree whether to adopt such "rulings" or not).
EDIT (Jan. 16): Correctness edit in view of changes to the Magic Tournament Rules with Rivals of Ixalan.
EDIT (Mar. 4): Edit last sentence of second paragraph; apparently the rules do clarify the situation with permanents not normally flip cards.
EDIT (Jan. 2, 2019): Edited.
EDIT (Jul. 9, 2019): Some rules were renumbered with Core Set 2020.
In this scenario, Leeches still deals as much damage as poison counters you lose due to that spell (which can be greater than 0 if you had poison counters before Melira or Solemnity entered the battlefield) (note that under C.R. 120.4, there are several parts to dealing damage), but even though Phyrexian Unlife makes that damage be in the form of poison counters (C.R. 120.3b, 609.4), you don't get poison counters this way because of Melira or Solemnity (C.R. 101.2, 101.3).
EDIT (Dec. 30, 2019): Edited, including because some rules were renumbered with Core Set 2020.
EDIT (Sep. 30): Correctness edit in view of recent rule changes.
Moreover, Languish doesn't target anything ("all creatures" doesn't indicate a target) (C.R. 115.1a, 115.10a), so Slippery Bogle can be affected by Languish despite its hexproof ability (see its Oracle text [C.R. 108.1]) (review C.R. 702.11b).
EDIT (May 3, 2019): Some rules were renumbered with Dominaria.
EDIT (Dec. 2, 2019): Some rules were renumbered with Core Set 2020.
EDIT (Jan. 17, 2020): Correctness edit.
EDIT (Feb. 19, 2020): Add rule citation.