I think you are right with Neville. Ever since the last book, I thought there were bigger plans for him. I thought Amelia Bones may have taken the locket and she goes by Amelia when it was actually her middle name. That would at least cover the last 2 initials. It may be Regulus Black, which would explain mention of a locket that no one could open. Although, they did say that Voldemort actually killed Amelia Bones himself.
I think DD is really dead, and Fawkes will somehow go to Harry. I was sure he died when Harry could move. It sure is fun to speculate on things though! After awhile though, I need to get out the tinfoil! LOL!
Wow!! That is a great theory.
Madame Bone's full name is Amelia Susan Bones. Her niece Susan was named after her aunt's middle name. So her initials are AMB.