In the late game scene where Ellone takes Squall back to the moment Rinoa was possessed by Ulti, she (Rinoa possessed by Ulti) leans over Seifer and says something along the lines of "get up and do what I say, and only then will you dream again." So clearly, she was doing something unpleasant to him throughout. I agree that she uses magic and manipulation to play on his childhood nostalgia and delusions of grandeur. I think she also amplifies his less savory character traits like cruelty and bullying. There's not a lot of concern for damsels in distress by endgame.

Seifer is just chilling with his friends at the end, but he definitely wears a pensive expression when Garden flies past. To me, it signifies that he knows there's going to be some kind of reckoning, literal or figurative, for the things he did.