I completely agree with you about DD and Snape--it couldn't have been just a matter of "Sorry I sent the Potters to their deaths, I'll be good from now on, scout's honor!" DD does have a fault in that he always sees the good in others--that's how he knew that Malfoy couldn't (or, at least
shouldn't) kill him. I do not think that DD was naive enough to take an apology from Snape and leave it at that.
So, if they did make an Unbreakable Vow or something similar way back when Snape returned repentant, who sealed their vow? My money, if that is the scenario, is on Lupin.
I can't imagine DD pleading FOR his life from Snape. DD had no fear of death; even in book 1 he referred to it as the next adventure for Nicholas Flame. I think the whole book was about passing the torch to Harry. Remember when he came to the Dursleys to make them promise to take Harry back for a brief time in the summer? Until he became of wizarding age at 17 and could go off on his own? After all HP has serious bucks and even that ratty house now. DD had to be telling Snape to really kill him. He'll still be around in his portrait.
Now what good are the horcruxes? How does V get out of a locket or ring to do anything?
It could have been Hagrid. He never accepts that Snape is evil when it is suggested to him, although that could just be because Dumbledore has such faith in Snape, and Hagrid adores Dumbledore.