The howler was related to sending Harry from the house. The letter with baby Harry must have explained the "Mother's blood" protection in some way.
What I don't quite get, is what induced Petunia to agree to raise Harry in the first place? Love for her sister seems the only answer, and yet Petunia was so bitter.
I agree, I kind of wish the Dursleys had reconciled with Harry, but they were so vile. I liked how Dudley showed grudging respect for Harry. It would have been nice to have Harry find the letter left with him or something so we could have seen what was in it.
The Epilogue was a little disappointing. I would have liked news on all the characters at the end.