Again, given what we've seen in FFVII and AC, it's fair to say that this human Jenova's cells infected is all that there is of her. Sephiroth's will is the will that ruled Jenova's cells. He was the only Jenova there was at that point, and he made a habit of using those cells as casually as he would his own body: in fact, he used them as his own body on numerous occasions. That’s another thing that is dependent highly upon your own beliefs. If you take a wooden boat and replace one plank from it every 10 years for 1000+ years until it is the same form but of different wood, it would fall to your interpretation whether it is the same boat.
We need to cut to the heart of this matter. I don’t want this to degenerate into two people endlessly repeating and reaffirming their view at each other. God knows there’s enough of that on the internet. I actually don’t think our points of view are that different. I think the answer to my question of what of Sephiroth was inherent in Jenova’s cells was the same as your’s: the mimic/mutation ability. The only difference is the pathway through which this occurs; you perceive it to be from Sephiroth to the cells, I perceive it to be from the cells to Sephiroth.