I think the important part that Jeebus misses in the discussion is that Dominion does not follow a strict "We have an event, we note down every ability that triggers of that event, then have other rules determine how those noted abilities resolve at some later time." paradigm.
I wish it would, but it can't due to the classic Secret Chamber/Moat (maybe now Diplomat/Moat) interaction.
Instead we have a model where after an event happens, there is a window to handle triggered effects for that event. If there are none, that window is over. If there are multiple, they are handled, one at a time, until there are none left.
The thing that differentiates it from the strict paradigm above is that until the list of pending triggered effects has become empty, new ones can be added to that list.Again, that is necessary for Diplomat/Moat. It's also been true for "At start of turn" effects: I can play a Hireling with an "At start of turn" effect (say Ghost or Piazza), and it's own "At the start of each of your turns..." effect will already trigger and resolve this very turn. It's why if you use Innovate on gaining Duplicate, you can call the very same Duplicate you just played on it's own gain Event. That wouldn't be possible if the allowed effects would be restricted to abilities that were available when the event happened.
For that reason, I think the difference between Model A and Model B is non-existent.
For example for "when you play, after (after resolving)" effects, it is clear that their window to be handled is after the card has done its on-play effect. Taking the above into account, it doesn't really become important when those abilities trigger, if in Model A in step 1) or in Model B in step 5).
What's important is that they exist at step 5.That's why I also think that the analysis of Jeebus about Champion is incorrect:
Champion has been ruled in different ways, but was ruled as “after resolving” until recently. Going by that ruling, it would work differently depending on the model. With model A, its own when-play ability (“+1 Action”) would not trigger upon it being played, since the ongoing ability would not be active yet in step 1; it would activate in step 4. Since it was not triggered, it would not be resolved in step 5. With model B, however, the when-play ability would trigger and be resolved in step 5, giving you one more Action.
As I said above, "when does the ability trigger" isn't relevant at all. What's relevant is that the ability exists at some time when the window for handling an event is active (not yet empty), which is true for both models.
The way I see it, the model for playing a is card as follows:
1) Annouce playing the card and put it in play (if appropriate).
2) All things that do something "first" when a card is played now have their window to be handled (Kiln, Urchin, Champion, Adventure tokens, Reactions on attack (Moat, Diplomat, etc.)).
3) All things that replace "following a card's instructions" apply now (Enchantress, Ways, not sure about Lantern). Only one can apply, since afterwards you are no longer following the card's instructions, even for the Way of the Chameleon.
4) The card does it's on-play effect or alternate effect (if step 3 replaced following the card's instructions).
5) All things that do something when a card is played (no "first") now have their window to be handled (Royal Carriage, Citadel, Sauna, ...)
For your examples, I already discussed Champion and I don't understand the Enchantress example.
For Sauna/Silver/Merchant: In step 5) of the Silver play, initially only the Sauna effect is pending. After it has been executed the Merchant has been played as part of it. Assuming it was the first Silver play, this means there is now a new ability in step 5 of the Silver playing, which gets handled and results in an additional +$1.
Summing up, it seems that my results agree with your Model B, but I don't think the reasonsing is the same. Thinking about when an abilitiy triggers is ultimately not relevant, as it is important only if it exits at some time during the window to handle the corresponding event.
or
anyway.
