Was talking to Theresa about it the other day, came up with a way the whole series could have easily been avoided...
All right, what it boils down to is that Voldemort gave Harry protection and Horcrux status when he killed Lilly. As Theresa pointed out, Voldemort offered to spare Lilly if she'd just step aside and let him kill Harry. Considering Snape's love for Lilly, and the fact that it was Snape who ratted out to Voldemort about where the Potters were, it seems probable that this was out of respect for Snape, perhaps a small reward for him if things went well.
As we all know though, she wouldn't just give up Harry, and so ended up dying. However, one small spell could have solved all of it, and I don't see where he'd have been above using it, either.
Imperius curse.
Cast it on Lilly, force her aside. Kill Harry, who now gets no protection from his mother, then give Lilly to Snape as a love doll. Snape gets his woman, Voldemort rules the world, no Harry to get emo in Book 5, everybody's happy!
hehehe...