By 'overpowering' the human cells, I meant that being half-human didn't affect the Jenova powers within him any. Sorry if I didn't get that through the first time. If he had more Jenova cells than yes he would look like a freak. And since the cells were introduced into him while he was still in the womb, the cells/DNA were woven into his own DNA. So he is thouroughly half-human. Otherwise, he would just be human with an alien arm or what not. Being affected by cells and being connected with cells are completely different.
Phsycologically, Jenova was his mother. Not whatsherface. So when he dicovered everything about Jenova, he automatically assumed she was his mother. That's when he wentMy moms Jenova which means I'm the chosen one!