Your argument is articulate but ultimately circular. The question is not the mechanism of DNA but the question of what constitutes a "someone", especially, in this context, a "someone else". You can and have argued that the uniqueness of the product of conception defines it being a 'someone', and since that DNA is unique and different from either parent, obviously a "someone else".
Without the benefit of our modern detailed knowledge of cellular stages, women, and to some degree men, have been living through this problem, about which you and I are merely theorizing, for millions of years. I have no idea what the subtle intimate signals are like, experientially, any more than I know what it is like to have a period every month. I am sure it is a rich and subtle experience from which I am almost completely excluded, alive with subtle signals, sensations, and intuitions.
I do not have any deep confidence that the thread of scientific evidence you have selected to measure with is truly a viable criterion. I, for one, am not willing to impose it on someone with an infinitely greater, more intimate, connection with the question then I can possibly have, and I think trying to nail such a profound question down to biochemical tracery is presumptuous. There is (I believe) a great deal more to the question of "what is a someone" than we have touched on.
It is possible that being outside the problem in the way we are, and knowing as little about the full dimensions of it as we do, we are condemned to being Jesuitical theorists, sitting around the plaza trying to adjudicate rights and wrongs of life in abstract terms while being actually unable to imagine the experience of the bright, vibrant young women passing by, who are actually living and walking in every instant the life we are dissecting intellectually.
So I think I will fold my hand, and go sit in a cave.