I really liked The Beginning as a title, though I was completely unsurprised by it. It's cliche, but it does fit and they made it work.
I agree with Visser19 about PTSD and everyone having varying reactions. This also goes back to what I said about wanting to be happy vs. making yourself unhappy. Jake dwells so much on his mistakes and "failures," he can't get past that unless he's trying to redeem himself, so yeah he's more messed up than the others in the end. Cassie, though, wants to move on. She wants to help usher in this new era, and maybe that's her own way of coping. So she heals. It probably takes a while, and that's why I think she doesn't really help Jake until later, when she's dealt with her own issues first. She knows she can't magically fix him, and as I've explained elsewhere it makes sense to me that they don't end up together. I don't think they just forget about each other at all, though. They simply walk different paths.
You have to remember Tobias spent the whole series struggling with his hawk vs. human dilemma and leaning towards the hawk side. He was always so uncomfortable in human morph. After the war, he might have finally become fully human again for Rachel, but without her? How exactly do you expect him to move on? He'd need a major push, from someone with a whole lot of dedication, and no one else is capable of giving him that until Ax goes missing.
I always thought Tobias' sudden fixation and then apparent disinterest in Loren was weird. The way I make sense of it, he was more focused on the concept of family than on Loren herself. The idea of a mother who cared about him got to him, kind of like the idea of a cousin who cared did. But he didn't exactly have a lot of time to bond with Loren before Rachel died, and I can see him turning away from Loren after that because she wasn't really important enough to be able to help him. I don't think she would know how. And Tobias already had issues with forming personal bonds with people, after losing Rachel I can see him resisting close ties with others.