I'd still love to hear an explanation and reasoning for what happens if A plays multiple possessions, and then forces B to play Outpost on the first Possessed turn.
Okay, let's reason this out.
Turn A:
—Alex plays Possession. This stipulates that turn P1 should follow turn A.
—Alex plays another Possession. This stipulates that turn P2 should follow turn A.
End of Turn A. Now two things are trying to happen at the same time: turn P1 and P2. Alex chooses P1. Therefore turn P2 will follow turn P1.
Turn P1:
—Becky plays Outpost. This causes her to draw 3 cards in cleanup, and stipulates that turn O1 should follow turn P1.
End of turn P1. Now two things are trying to happen at the same time: turn P2 and O1. Alex gets to choose, because Becky is being possessed.
...If Alex chooses O1, Becky will play her own turn with a three-card hand, and turn P2 (with Alex possessing Becky) will follow, with a five-card hand unless an Outpost is played on turn O1. Turn B will follow turn P2.
...If Alex chooses P2, Alex will play Becky's turn with a three-card hand, and turn O1 never happens because it would be a third consecutive turn. Turn B will follow turn P2.
Does that sound right?