I think Oda this time is not lying to us and actually revealed the truth: the Will of D. is the Will of Davy. A "will" by definion is something that a person own,not an entity,so its more reasonable than the "will of Dawn" or other similar things. About the double name,lets remember that the public name of Xebec is Rocks,so the D. is useful to signal that altough he doesnt reveal his real last name,he carries the will of Davy anyway.
Also,i dont think the D. is really a part of people's last names. Its something that the D. people put to themselves,and anyone can obtain it. The scene where Ace and Luffy give the D. to Sabo,altough its potrayed as a playful scene,may actually reveal how it works. If you want the will of D.,you can have it