[@Jangel13]you're more or less on the right track. Don't be related to gasper. It just felt like necessary information to know that there is a clan/lineage in the dxd world who's existence bares reference to a figure in history who you are utilizing. Still, this is an au. So you don't need to necessarily worry about them specifically. I do recommend you look up the vampires of the dxd universe, and learn as much about them as you can because that's what I will do myself to keep things to an extent in line with some of the lore and world built.

Still in order:
Yes, longer line of lineage. 
Cause of death should fit the times as they are. 
And it should make sense how or why tsumiki came across and reincarnated him, within I would say the first half hour or so of death as a devil can only reincarnate the living, dying or recently dead (to my knowledge).