This great!!!! Thanks for translating it. I think that Death is just a powerful demon that feeds on the souls of the dead. I doubt that he is actually the literal angel assuming that one actually exists in the CV universe. I also think that, while it was left open, Curtis wasn't fully dead when Death took control of the body. Curtis was likely at that point where he was right at the edge. Sort of dead, but not dead yet. In other words, the kind of temporary death that you can be revived from is you receive medical attention fast enough. In this case, the potion that Hammer gave him. Keep in mind, at that point, Death had likely already entered his body and his soul was suppressing Curtis's.
I also think that the reason Alucard didn't come to the castle is that he was in Japan keeping an eye on Soma. I mean if you were Alucard and Death approach Soma trying to coax him to become the master of the castle, would you leave him unguarded or would you stick around to make sure he nothing happened to him?