I would LOVE LOVE LOVE! Then to do a sibling relationship parallel with them, I didn't even consider that and now I really really want it! I think it would make a fresh change to the "losing a lover" parallel and could really bring these two a lot closer.
I also believe Bellamy can forgive Echo, the guy is very forgiving, he forgave Clarke for letting a bomb drop on Octavia
plus like you said there are other contributing factors and these are extreme circumstances. And maybe she can play a part in helping him forgive himself, maybe they can mirror it through Echo, it makes sense for someone on the opposite side of the war to help him with his redemption, I think that would make it more powerful.