Isn't there an entirely different woj(s) that says that the reason harry didn't keep Kravos' power was because there was no death/soul exchange? that you are what you eat, even if your a wizard? Because I always thought it explicitly implied the key to taking in and keeping power is death/soul infusion? same way the darkhallow works, stone table, ect. I assume because soul is they key to change, in order to have a proper fusion requires said key.
& I assume that having a soul mixed into the mantle is how a mantle is formed as a separate thing. not that it necessarily stays 'soul' as defined by human usage. but mantles seem to have their own 'spirit' if not consciousness.