how did the spirit fix that part of our genomes so he could be a perfect "human"?
Ehh you'll prob think this a cop out but fetal Jesus was created by Jehovah, he wasn't the result of a human conception. So if you can accept a god can jump start the birth process and impregnate a virgin you're gonna have to swallow the fact that he can also chuck out the corrupt DNA from the raw material he used, that being one of Mary's eggs. The resulting foetus was then like an empty house for Micheal to move into. The bible says spirits can possess bodies and the examples given show that problems arrive when two personalities try to inhabit that same body. In Jesus case he would have moved into an empty vessel.