MTG Salvation

Yes, pefectly legal, and in fact you have several opportunities in the scenario as you described it. First, you can respond to the creature spell your opponent has cast while it is still on the stack. Second, you can let it resolve and activate the Flagship afterwards. No step or phase where players get priority ends until all players pass priority in succession. Just the stack being empty is not enough. And having passed previously is irrelevant if someone else is doing something. Then the whole chain of passing has to start anew. So theoretically, players can cast spells and activate abilities, and resolve them, then do it again, in an endless fashion all during the declare attackers step. Essentially, all players have to agree to move to the next step, and the stack must be empty at that time, for the game to move forward.

The phrase in 704.3, that the game '[...] then performs all applicable state-based actions simultaneously as a single event', is certainly necessary to clearly provide what it means to do (possibly) two or more of the State-Based Actions when applicable. Some notion of timing is needed.

If we consider the case of "simultaneity" used in spell/ability instructions, we know that this word appearing on, e.g., Breaking Wave, is telling us to comprehend a set of transformations before applying any of them. But, replacement effects behave here as anywhere (let's look at Goblin Welder) and may replace some part of this set of transformations with a sequence of transformations. The procedure that we use to resolve Goblin Welder's ability is not unusual here. We're told to do some things, the original event is replaced with a different collection of events, and the new events might have a sequence.

Naturally, the unreplaced part of Goblin Welder is co-ordinated to the timing of the original counterpart of that instruction. The event that was 'spliced in' is also co-ordinated to that fraction's timing. Let's say the sacrifice is replaced by the Ravenous Slime here. There is no reason to struggle for a theory of space-time that can make the putting of Welder simultaneous to the sequence that replaced the sacrificing. The simultaneity only told us how to comprehend or configure the event we were originally supposed to consider.

State-Based Actions are also phrased as instructions. Events arise from instructions of any breed - I do not know of a rule that would let the SBAs behave differently from spell instructions in regard of the above reasoning. So when (as is the oddity from a few months ago) Lich's Mirror replaces losing the game, it isn't cramming that list of instructions inside the timing of the losing Action. It is just replacing the losing Action, and now stands beside the instructions which were imposed by another SBA (so yes, it is effectively like they 'begin' simultaneously).

So Ravenous Slime tells a player to put counters on Ravenous Slime; this is after the exiling happens (because of separate verb)*. Since Ravenous Slime is already in grave, this instructions is ignored because it is impossible (CR 614.6).

*this is a tricky point that I would have missed if I jumped in to answer this thread when it first appeared. The putting is not simultaneous to the exiling, which I am not used to treating as relevant.

The life gain from Samite Ministration is due to a triggered ability (C.R. 615.13), not from a spell, so it won't make Firesong and Sunspeaker's last ability trigger. See also this thread.

In the case of Chant of Vitu-Ghazi, the damage prevention effect happens at the same time creatures would deal damage (C.R. 615.5), so even while a white instant or sorcery spell is resolving (e.g., Repentance). It remains to be clarified whether the phrase "a white instant or sorcery spell causes you to gain life" means "you gain life during the resolution of a white instant or sorcery spell" (compare Firesong and Sunspeaker with Karmic Justice or Equinox).

EDIT (Apr. 18): See comment 4.

You can use Plummet, Murder, or Naturalize to kill Archetype of Imagination. As long as it meets the targeting requirements of the spell, additional types don't matter. Is it a creature? Yes. A Creature with flying? Yes. An enchantment? Yes.

Currently both Madness and Conspire use similar wording and they do involve separate triggers. So being a separate trigger it is different in that you don't choose a target until you sacrifice a creature.

Though being a new card it is possible that it is a new template and functions as though it was "If you do", it is unlikely.

Dragonfox will be missed here. My heart and blessings go out to his family and all who loved him.

Yeah, that works fine. Conjurer's Closet Exiles the creature so Whip's replacement effect doesn't apply. Then, you can choose to put that creature in the Command Zone as a replacement effect so Conjurer's Closet will return it to the battlefield.

Reading through the steps, and how Replacement Effects get applied, what actually happens is this:

Two triggers happen at the end step.
Whip of Erebos is put onto the stack first followed by Conjurer's Closet.
Conjurer's Closet resolves and attempts to exile the creature.
This bypasses Whip's replacement effect since it is going to exile.
You choose to put your general into the Command Zone.
Now, Whip's replacement effect tries to apply (since it is now going to the Command Zone instead of Exile).
You then choose to put it into the Command Zone again instead of Exile.
Conjurer's Closet tracks it into the first public zone it goes to (Command Zone) so it is returned from there.
Whip's trigger resolves and does nothing since it is now a different object.