Oh, I've been in multiple fandoms (like most versions of Trek) where one character ends up sleeping with an AU version of himself. In fic, I mean. I don't know of any that went there in canon.

That's a good question about despair. In Christianity it's usually interpreted as the unforgiveable sin against the Holy Spirit that's mentioned in the New Testament, but I'm not sure it occupies such a prominent role in other religions.

I like the redemption angle too. And both of them clearly feel the need for it--Dean is still haunted by what he did in hell--so it would be great if that turns out to be what lets them save the world.