Found an even bigger plothole: Death shouldn't exist. Why? Because: At the end of PoR, Dracula absorbed and destroyed Death. Therefore, Death shouldn't exist.
Death in the Castlevania series is essentially a series of physical avatars for the actual force of Death. Since Death cannot be killed, as he is a fundamental part of the cycle of life and the universe (or implied in LoI that he has 'divine powers,' indicating a connection to God which also puts him high on the no-kill list), all that can be done is destroying his physical avatars and thus weakening his ability to manifest himself for a time. I don't think Death just twiddles his thumbs while Drac's dead, he still has to go about his business everywhere else; because of this, I theorize that Death has access to countless physical avatars all over the world, but he reserves the strongest puppet for when Dracula is alive. With this idea in mind, it can be assumed that this super-body takes time to build up in strength, and this is what Death does during Drac's periods of 'death.' Once that super-body is destroyed by a Belmont or whomever else, it can't be whipped back into existence as Death's theoretical 'common reaper' avatars can. It takes however long is necessary to recreate that powerful avatar, and Death stows it away for the day his bro wakes up again.
This theory could also be tied into Death being an agent of Chaos, since death as a process, and the decomposition that comes with it, could be considered a form of entropy. Entropy being an essential part of Chaos, Death could potentially be an agent of entropy, and thus due to THIS connection with an even more fundamental part of of the universe, Death can't truly be killed.
I think Dracula absorbing his 'power' in PoR isn't so much him absorbing Death's fundamental soul and essence (yes, I know he says "Soul Steal" when doing it, but that's just the name given to that ability), but the immense power and fragmented soul-of-sorts contained within that super-body.
tl;dr you can't kill Death. Unless Abdul Alhazred is involved.