The Tournament Rules don't really cover etiquette. There's no particular infraction for reaching for the sideboard when you're controlling the player who owns it, though it may be considered rude by some people.
Also, to clarify for anyone still wondering, it's been agreed upon by judges that we will not force a player to reveal the sideboard if the player concedes immediately upon the request. Since the game is over as soon as the concession is made, there is no longer a discrete presented deck and sideboard, and the in-game effect granting control of the player ended as well. Plus, it can get very touchy if you try to force a player to physically do something with their property and could create legal concerns, so concession is an acceptable way to avoid revealing your sideboard.
X in a mana cost is considered 0 unless it is on the stack as a spell.
Not exactly. The casting cost is XX, but the converted mana cost, the one relevant for Abrupt Decay, is 0 at all times.
While not relevant for Abrupt Decay since it can only target permanents, an X in a mana cost for a spell on the stack is treated as the value chosen for it while casting. So Chalice with X=2 while it's a spell on the stack would indeed have a converted mana cost of 4. Once it resolves and becomes a permanent, it has a converted mana cost of 0, because the Xs in the mana cost are treated as 0 when anywhere other than the stack.
No, it's "faster" than a state-based action. The "this is an aura" effect ends the very moment the thing it's attached to leaves the battlefield. Though if it was a state-based action, the answer would be the same, since state-based actions are checked just before triggered abilities are put on the stack, which is just before a player would get priority.
"Can't" always beats "can" in MtG. So Chalice will trigger, and the trigger will try to counter the spell. When it does, Cavern's ability says, "No, that's not even possible." So the trigger does nothing, and the spell resolves normally.
I don't see a creature dying between step 4 and 5, so now sure how you reached step 5 at all. Before damage, Sengir Vampire has one damage marked on it from Ashmouth's ability, then you add 1 more damage from Vithian Stinger. That leaves a 4/4 with 2 damage marked on it, and no creatures have died.
One Ashmouth Hound and Sengir Vampire deal damage to each other, both die at the same moment from lethal damage (this triggers Sengir Vampire's ability because it died at the same moment as the Hound, though that trigger is irrelevant since it can't add a counter to a creature no longer on the battlefield). This triggers Butcher, and you sacrifice Stinger. Stinger's death doesn't trigger Sengir, because it's already been dead for a while now.
Putting a land onto the battlefield from a fetchland or any other instruction that doesn't use the term "play" for the land isn't playing a land and doesn't count toward this rule. That particular clause is referring to other effects such as Djinn of Wishes, which could sometimes tell you to play a land, but you'd be unable to if you've already met your maximum number of land plays for the turn, or if it isn't your turn as the ability resolve.
The Zegana would enter the battlefield with some number of counters (assuming you have another creature), as that's a replacement effect and not a triggered ability. Zegana's ETB ability (to draw cards) would trigger, Flash would finish resolving and you sacrifice Zegana. Then Zegana's trigger goes on the stack. When it resolves, it will use the last-known information about how Zegana looked on the battlefield, and you'll draw cards equal to the power it had when it was sacrificed.
2 Headed Dragon isn't an officially recognized format. I can only recommend that you use the 2-Headed Giant infect rules, which is 15 counters for the team to lose the game. Since 2HG uses 30 as a starting life total (1.5x the normal 2-player starting life total), it's reasonable to use 15 poison (1.5x the normal 2-player poison count to lose the game).
Beyond that, I can only say that you should agree on a number with your playgroup, since the rules don't provide an answer for you.
Not quite. You have to put both evolve triggers on the stack when the other creature enters. Then one of those has to resolve first, triggering the "when this evolves" trigger, which has to go on top of the stack. That trigger will only add a counter to other creatures that already have a +1/+1 counter (not your second Krasis without counters at this point). Then the second evolve trigger will resolve, triggering the other "when this evolves" trigger, and adding a counter to the first Krasis. You end up with a 4/3 and a 5/4.
Both rulings can be correct. When a trigger is resolved incorrectly, that's a game rule violation (as opposed to a missed trigger). The possible fixes for a game rule violation are to do a full rewind to the point of the error, leave the game as it is (no rewind), or performing one of the 3 prescribed partial fixes (none of those apply here). So neither ruling was wrong. The question becomes whether the disruption to the course of game is greater by rewinding or by not rewinding. The judge has to make that decision.
Yes. A spell isn't countered by the game rules for having no legal targets until it would start to resolve (both players pass priority without doing anything while it's the top object on the stack). Up until that point, it can be targeted by Remand normally.
Yes it's possible to Chord for 5, using the Birds for either mana or Convoke yields the same result. You announce Chord with X=5, and total cost becomes 5GGG. You can pay that cost using your lands for GG1, and tap 5 creatures to pay for G4, or use lands and Birds for GGG2, and tap 3 creatures to pay for 3. The only thing you CAN'T do is use Birds as both mana sources AND tap them for convoke, since mana abilities must be activated before paying costs, and if the Birds are tapped when you go to pay costs, they can't be tapped for Convoke.
You can't interrupt the layers. The game does a full layers check before determining the final characteristics of any object, without using the stack. So Humility enters, and the game goes through the full layer system to determine the characteristics of each creature. Humility makes the creatures 1/1 in layer 7b, Zealous Persecution gives them -1/-1 in layer 7c (and any static abilities giving +X/+X also apply in this same layer), counters give the creature +1/+1 (each) in Layer 7d. The end result after applying everything is all that matters. Any creatures with a +1/+1 counter or static P/T boost will survive.
Also, to clarify for anyone still wondering, it's been agreed upon by judges that we will not force a player to reveal the sideboard if the player concedes immediately upon the request. Since the game is over as soon as the concession is made, there is no longer a discrete presented deck and sideboard, and the in-game effect granting control of the player ended as well. Plus, it can get very touchy if you try to force a player to physically do something with their property and could create legal concerns, so concession is an acceptable way to avoid revealing your sideboard.
While not relevant for Abrupt Decay since it can only target permanents, an X in a mana cost for a spell on the stack is treated as the value chosen for it while casting. So Chalice with X=2 while it's a spell on the stack would indeed have a converted mana cost of 4. Once it resolves and becomes a permanent, it has a converted mana cost of 0, because the Xs in the mana cost are treated as 0 when anywhere other than the stack.
One Ashmouth Hound and Sengir Vampire deal damage to each other, both die at the same moment from lethal damage (this triggers Sengir Vampire's ability because it died at the same moment as the Hound, though that trigger is irrelevant since it can't add a counter to a creature no longer on the battlefield). This triggers Butcher, and you sacrifice Stinger. Stinger's death doesn't trigger Sengir, because it's already been dead for a while now.
Hope this makes sense!
Beyond that, I can only say that you should agree on a number with your playgroup, since the rules don't provide an answer for you.