Okay, these are some pretty good points. I'm still not completely convinced.
I'm pretty sure the Mandarin interaction is correct. You don't choose the two actions before you gain either of them, you just gain an action costing the amount you overpaid, twice. "Gain 2 Actions" is just shorter than "Gain an action, then gain another action."
Discarding and trashing are atomic, meaning you discard or trash several cards all at once, not one at a time.
Drawing and gaining are sequential. So you draw or gain a card, resolve any effects from doing so, then draw or gain the next one, etc.
Could any of you back this up?
As far as I know there is no way to gain 2 cards at the same time pre-Guilds (even with Talisman or Border Village you gain the cards in different phases. For example with Talisman you gain the Talisman card on-buy which happens before you gain the card from buying it) [EDIT: Okay this is wrong, there are plenty examples, like Trader, two Talismans, Mountebank, Develop, ..., but see my next comment below]. So pre-Guilds it didn't matter whether gaining is atomic or not. So how do you know whether it is? Is it somewhere in the rulebook or has Donald posted it somewhere?
Besides, your interpretation- "Choose two Actions, then gain one, then gain the other" would mean you could never gain two copies of the same card, since the card doesn't "see" that there are Action cards eligible for gaining below the top card of the pile.
This is wrong, even if my interpretation is correct. In my interpretation you can choose - say - "Village, Village", then gain the first Village, and then gain the second Village. It doesn't matter that there's a different Village on top the second time. Otherwise Talisman wouldn't work: when you buy a village when Talisman is in play, you first gain the Village from Talisman, and then gain the Village you bought, which wasn't on top of the pile when you chose to buy village.
Edit: As for the relation to trashing effects- you're not choosing all the cards to trash, then trashing them one at a time, you're trashing them all at the same time, then choosing the order their on-trash effects resolve. I realize that's a subtle distinction but the point is, you never gain more than one card at a time in Dominion, and if one card has multiple on-gain effects to resolve, you resolve them one at a time in the order you choose.
This is a very good point; I made a mistake there. You indeed trash all cards at the same time. (although I think my version works the same for all official cards, it would be different if there was some card which has an on-trash ability like "When you trash this, discard your hand")
From the guilds rulebook, stonemason :
The cards you gain are gained one at a time; this may matter with cards that do something when gained, such as Inn from Dominion: Hinterlands.
This doesn't state explicitly whether you choose the second card before or after your first gain. Even in my interpretation above you gain the cards one by one. And by the way - this is in the description of the on-play part of Stonemason...
I'm starting to think that you guys are right, but if one of you could present a better indication/proof of your point to erase all my doubt (maybe Donald has commented on it somewhere or there is some rule), I'd be grateful. I still find it intuitively strange that gaining and trashing work differently, because somehow trashing is atomic and gaining is not (okay, in the rules it says explicitly that trashing is atomic, and it doesn't say the same about gaining. But that doesn't explicitly say that gaining is NOT atomic).