I hadn't actually thought about Seifer. I reckon he would have returned to Balamb eventually, because that's all he ever knew. It would also provide a renewed link to Rinoa which could explain why Ultimecia used him so heavily.

I actually do think the relationship between Squall and Rinoa would last, at least as long as Rinoa doesn't give in to her powers. She was the first person Squall could ever let himself care about, I think he'd hold onto it. I also think he'd be true to his word nomatter what his feelings were.