Very detailed, illuminating analysis and discussion, Ellen, thank you for starting it.
I agree with Xie that, of the two, Brian is the character who seems more "different" from what we're given in the show, albeit remaining completely canonical (like Justin).
His active pursuing of Justin's friendship, his emotional openness - in contrast with Justin's hesitancy - make perfect sense, though, not only because we're effectively in a post-S4 (thus post-cancer) AU, but especaly because in this AU, he's painfully lonely. His going to New York has not only meant the end of his relationship with Justin, but also a serious weakening of his ties with his friends and family in Pittsburgh, not all with positive cosequences.
Justin's arrival in NYC gives Brian a second chance in love, but also an opportunity to reconnect with his friends, as we see when they go back to Pittsburgh together.
Justin seem perfectly IC to me, just hardened in a different way than in the show, because the defining event of his youth hasn't been the bashing, but Brian leaving him. Unsurprisingly he's become more disenchanted and much faster than in canon about romantic relationships, but this happens also in canon, if more slowly and by a different route.