Just a warning: I know this post is kind of gratuitous and only for legal nerds.
I want to point out that this thread is interesting and quasi-important. Here's my reason: As far as I can tell, DXV's ruling here sets a precedent, rather than just serving as a logical answer to a question where someone simply hasn't thought through all the logical implications of the existing rules.
In my reading, the original question -- are fortresses trashed during possession set aside as "trash" or sent back to hand? -- is not already determined in the rules. My reasoning is that unlike almost any other decision that Possession allows the possessor to make, this particular decision (whether to return to hand or set aside Fortress) was never a choice the player would have to make if he/she were not being possessed.
Possession's card text says that the possessor will "make all the decisions for him," i.e., the possessed player. But this wouldn't be a decision that the player could make unless he were being possessed, so it's not clear that the possessed player would make them to begin with. That is, since we don't know who would make that decision in the non-possessed case (because it's impossible in the non-possessed case) we can't say the possessed player makes that decision, and therefore we can't then assign the actual decision to the possessor.
I can't think of a parallel situation, but I'd be interested to hear if anybody can come up with a similar case, especially one on which DXV has already ruled. Or if they just plain disagree with me.
I suppose my point is, in case future Dominion scholars are studying the legal framework of the game, they should take note that case law has determined that "make all decisions for him" includes decisions that the player was never licensed to make in the first place but which have been forced upon him.
I know, I know...hair-splitting.