It's been intertwined in an immense amount of Xan lore that there has always been a prophet, typically someone of the Athna bloodline which is directly linked to Xan's. Prophet is really just a cultural rank, a figure that Paladins can look towards for guidance but it doesn't mean people can seek the very same from any other Keeper. This was a discussion I had with the main writer of Xan lore, Tsuyose, and its what he and I agreed on. It should just be noted that any paladin is capable of proclaiming themselves as prophet, there wouldn't be any ooc barriers for it.
I don't really know what you're specifying in this so I'll try to explain without being really wordy like I am in the lore. Person passes trial to become Keeper, person comes into Xans realm, Xan essentially rips their soul out and sets it to the pillar of binding within his realm in which their original body soon becomes defunct and placed somewhere for self keeping. Xan then creates them a new body within his own fashion, replacing crimson blood with his godly ichor and replacing normal bone with a crystallized version of his ichor that acts as a replacement. The soul is then linked from this pillar to the body which allows them to function. I believe this is very similar to how the current wraiths (or some version of them) works by having their actual souls bound to some black pillar in the abyss.
I'm unsure what material could be really used as preparation, perhaps a sacrifice of the Keepers blood in the chancery but that'd be seen as somewhat insulting to the eyes of Xan. Chanceries are the place where Paladins are connected, where one is the strongest and their connection to him is that much closer, allowing the Keeper to manifest a far stronger power in order to create a realm through an amalgamation of the mists and light to get there. The significance of a Chancery is not just so a Keeper can travel there, but its the holy of holies for a Paladin, where one would go to pray, feel closest to their aengul, and also defend to their very last breath.
Also with the mists, using it in combat was possible in the original Paladin lore. Any paladin could just create weaponry/objects from the mists to be able to used in combat. Personally I didn't think it was needed as a combat addition because it'd detract from the two paths of Vindicator and Wyrmstalker in the rewrite. I just liked it as a cool thing to be able to teach with and make some neat roleplay out of.