I don't think Justin was lucky. To me, going by their interactions in early season 3, they would have ended up back together anyway, even without Justin scheming. It seemed inevitable, and pretty blatant that Brian wasn't going to put up much of a fight.
Justin might have felt lucky that Brian was open to being with him again, but I don't get the impression that he felt solely responsible for the break-up, or that he thought Brian was generously "taking him back". It seemed pretty clear that they had both reached a point where they knew where they'd gone wrong, and they were choosing to start again together, with a new understanding between them of what they were doing. If anything, Brian probably thinks he's the lucky one, for Justin still wanting to be with him after everything.
I haven't watched this season for ages though, or QAF at all. I've just started watching it again recently from the beginning. I've done Season 1, and I'm just about to start on Season 2, but all I can think about it how much I hate to see all the post- bashing angst, and then the breaking up angst. I want to skip straight to Season 3 for the happy together-y love. But I won't. :p