If such a card ever existed, that's probably how they would do it. As far as I know, there isn't any precedent for that kind of effect, so we can only speculate about what the templating would be.
Strictly following the rules, this is how it should be done, but most players would opt for something more expedient.
For Flux: In APNAP order, each player chooses a card (or cards) to discard and puts it face-down on the table. All of those cards are discarded simultaneously. In APNAP order, each player draws however many cards they discarded. Then Flux's controller draws a card.
For Lore Broker: In APNAP order, each player draws a card. Then in APNAP order, each player chooses a card to discard and puts it face-down on the table. All of those cards are discarded simultaneously.
EDIT: Here's the rule for it, if you want it.
608.2e. Some spells and abilities have multiple steps or actions, denoted by separate sentences or clauses, that involve multiple players. In these cases, the choices for the first action are made in APNAP order, and then the first action is processed simultaneously. Then the choices for the second action are made in APNAP order, and then that action is processed simultaneously, and so on. See rule 101.4.
If an effect tells multiple players to draw a card (or multiple cards), the draws are done in APNAP order.
So the active players does all of his draws, then the next player in turn order, etc. Normally this doesn't matter, but in the situation you described, it definitely would.
Yes, if you animate them with Karn, they will be 5/5. Rite of Replication tokens have all of the copiable characteristics of the target (in other words, what's printed on the card), so they will have a converted mana cost of 5.
Note: this only works because the Doubling Season tokens are not creatures (Rite of Replication only creates Doubling Season tokens, unaffected by anything that would make them a creature). Karn specifically animates artifacts that are not already creatures.
Yes, that's correct. Eidolon of Blossoms has a triggered ability, so each token will trigger for itself and each other token (and the original Eidolon will trigger for each token).
The difference is that Eidolon has a triggered ability whereas Doubling Season has a static ability that creates a replacement effect.
Assuming you kicked the Rite of Replication, you get 10 tokens. Replacement effects, such as the one on Doubling Season, don't "look back in time" to modify events, so you only get the effect of the original Doubling Season.
Basically, triggered abilities will see other permanents that enter at the same time, but replacement effects have to exist before the event that they're replacing. Since the tokens don't exist until after the event of "entering the battlefield", they won't change it in any way. But the original Doubling Season will.
Without involving any other cards, there isn't anything you could do. After you return Glaciers to your hand, you won't get priority again until the Pit trigger is already on the stack.
There are some other cards that could arguably give you a better outcome (for instance Flicker the Glaciers in response to its own delayed trigger, so it stays on the battlefield), but that's kind of outside the jurisdiction of the rulings forum.
Yeah, that works exactly how you described. My only correction is that the legendary rule doesn't make you "sacrifice" anything, it just tells you to choose one and put the rest in the graveyard.
EDIT: And you're right that this wouldn't get you a slow play violation, because the loop is not conditional.
No. Targets must be chosen before any costs are paid. In this case, if there are no creatures in the graveyard, you can't even begin to cast Return to the Ranks, because there are no legal targets.
For converted mana costs, X is treated as zero unless it's a spell on the stack. So the Chalice, if it was cast for X=1, would have a CMC of 2 on the stack, but once it becomes a permanent its CMC is zero.
1) No, in fact in this case Grave Betrayal won't even trigger because the Colossus didn't "die". Dying specifically means going from the battlefield to the graveyard.
2) Sure, you can do that. Act on Impulse just gives you special permission to play those cards from exile. Everything else about playing them stays the same (for instance, you can still only play one land per turn, even if the second one would be from Act on Impulse).
1) Containment Priest's ability is a replacement effect, so it doesn't apply to creatures that enter at the same time.
2) No again, for the same reason.
Replacement effects have to exist before the event that they would be modifying. Cards like Containment Priest only have an effect if they're on the battlefield, right? And by the time they're on the battlefield, so is everything else that enters along with it, so it's too late to modify how they enter at that point.
Just for clarity's sake, for the second question: If you choose to put it on top of your library, that will still trigger Necropotence, but the trigger will fail to find the card in the graveyard, and end up doing nothing.
For Flux: In APNAP order, each player chooses a card (or cards) to discard and puts it face-down on the table. All of those cards are discarded simultaneously. In APNAP order, each player draws however many cards they discarded. Then Flux's controller draws a card.
For Lore Broker: In APNAP order, each player draws a card. Then in APNAP order, each player chooses a card to discard and puts it face-down on the table. All of those cards are discarded simultaneously.
EDIT: Here's the rule for it, if you want it.
EDIT AGAIN: Didn't read Flux. Fixed first answer.
So the active players does all of his draws, then the next player in turn order, etc. Normally this doesn't matter, but in the situation you described, it definitely would.
Yes, if you animate them with Karn, they will be 5/5. Rite of Replication tokens have all of the copiable characteristics of the target (in other words, what's printed on the card), so they will have a converted mana cost of 5.
Note: this only works because the Doubling Season tokens are not creatures (Rite of Replication only creates Doubling Season tokens, unaffected by anything that would make them a creature). Karn specifically animates artifacts that are not already creatures.
Yes, that's correct. Eidolon of Blossoms has a triggered ability, so each token will trigger for itself and each other token (and the original Eidolon will trigger for each token).
The difference is that Eidolon has a triggered ability whereas Doubling Season has a static ability that creates a replacement effect.
Basically, triggered abilities will see other permanents that enter at the same time, but replacement effects have to exist before the event that they're replacing. Since the tokens don't exist until after the event of "entering the battlefield", they won't change it in any way. But the original Doubling Season will.
There are some other cards that could arguably give you a better outcome (for instance Flicker the Glaciers in response to its own delayed trigger, so it stays on the battlefield), but that's kind of outside the jurisdiction of the rulings forum.
Tundra
Yes. High Tide doesn't say anything about whether or not the mana produced was U, only that the land that was tapped has the land type "Island".
Yeah, that works exactly how you described. My only correction is that the legendary rule doesn't make you "sacrifice" anything, it just tells you to choose one and put the rest in the graveyard.
EDIT: And you're right that this wouldn't get you a slow play violation, because the loop is not conditional.
Purphoros, God of the Forge
Bloodsoaked Champion
Return to the Ranks
No. Targets must be chosen before any costs are paid. In this case, if there are no creatures in the graveyard, you can't even begin to cast Return to the Ranks, because there are no legal targets.
Also, remember to use card tags.
For converted mana costs, X is treated as zero unless it's a spell on the stack. So the Chalice, if it was cast for X=1, would have a CMC of 2 on the stack, but once it becomes a permanent its CMC is zero.
2) Sure, you can do that. Act on Impulse just gives you special permission to play those cards from exile. Everything else about playing them stays the same (for instance, you can still only play one land per turn, even if the second one would be from Act on Impulse).
2) No again, for the same reason.
Replacement effects have to exist before the event that they would be modifying. Cards like Containment Priest only have an effect if they're on the battlefield, right? And by the time they're on the battlefield, so is everything else that enters along with it, so it's too late to modify how they enter at that point.