We know that the amulet of kings wouldnt just accept anyone if the dragon fires aren't lit because if you the player try to equip it while you have it, you get a message that says it slides off of you. So the theory about Mankar changing his nature with the Mysterium Xarxes is the more likely of the two.
🥹Omg you took my advice and added gameplay in the background, maybe this isn’t the first video with it i think i’m behind but would you take modded gameplay?
One thing to note about Mankar, he was at one point in possession of Mehrunes razor. It's said he used it to change himself from a Bosmer to an Aldmer or at least as close enough to one. Seeing that he used the artifact to change his race i wouldn't be surprised if he also gave himself dragonblood in order to wear the amulet