Hell YEAH, oh my chaos, that's so good. It could be both so angsty and sweet.
Kingsley as Widogast's monster. I can't think much about it, all my brain could produce is barking (and I have not read Frankenstein, so I don't know all of possible details of that kind AU's).
...Well, actually I can. Because I have an OT3 which I adore.
Would Essek help Caleb with that experiment? And would Caleb be devastated then he doesn't bring Mollymauk back, but bring Kingsley to this world instead? Would he do his best to help this new life to learn how to live? And what would other members of M9 do about it?
Or if he actually bring Molly back, wouldn't he be terrified of what he had done? Would he be ashamed of his decisions and how selfish he was to not letting go? And how would his lovers help him to get to live with that consuming guilt?
Molly's/Kingsley's learning sometimes would be funny and sweet. He have some trouble with his memories and at first times his coordination would be real shit. And by that I mean he would sometimes drop things, stumble on nothing and would be really in need of someone's support (maybe even walking and eating would be trouble, so then he managed to walk by himself and hold his spoon through the whole dinner it would be a small, but very important victory). I don't see either Molly or King as someone who would be scared by his new condition... Well, maybe a little, because oh dear he reeks death and graveyard (and maybe Essek could help it by some perfume? some matching perfume for all of them oh my chaos).
And in the end they would have a very fragile human wizard with whom they would be until he passed in his bed, all grey and wrinky, but so much loved. And then there would be Essek's turn he would take Molly/King with him to the other side (your honor, I hope they would be buried all three together, they deserved it).