As you know, I'm not opposed to a theory that posits a later jump time. The prospect of having to stop in the states to refuel seems to have shaken him up a bit, and him having to improvise a new plan explains how haphazard he seemed to be in the later stages of the hijacking.
Yet I do think it's logical and reasonable to assume that he wanted to jump back where he started. The only problem with that is that he didn't really create a scenario to where he could guarantee that they would be flying back over Portland. V23 and V27 were the only airways that headed south out of Seattle. V27 would be a pretty dangerous airway to risk them taking if you were wanting to jump near Portland. It hugs the coast and is sometimes over the water in that area around the Washington/Oregon border.
I've never bought into a Seattle jump because Cooper wasn't ready to jump when they took off. Tina says he was still messing with the bag when they were taking off i.e. he wasn't ready yet. If he wanted to jump in Seattle, it's reasonable to think that he would have been totally situated when they took off.
Regardless of where he wanted to jump, Cooper was certainly winging his DZ.