I agree with your main point (that this is a stumbling block for some people), but there are others who will contend that A and part of B (namely the irreversible error) do apply to unwanted babies (usually, or on average), and that the reason why abortion is more evil than contraception is because it's an error of commission rather than omission.
Killing adults is less reversible in the sense that if you kill comedian carlos mencia, you can't get a new carlos mencia if you change your mind. In contrast, babies are basically fungible.
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.