My view is that the central test for deciding whether abortion should be allowed is when can the rights of the fetus override the rights of the mother.
When answering that question I first think we need to look at the rights in question, in this case we see that we are dealing with the right to future life or (future bodily autonomy) and the mother's bodily autonomy. Clearly it is a case of two conflicting rights on the same thing. Next we have to see when would the being gain rights that could outweigh that of an existing being. I think that clearly just being an animal is insufficient, as when there is a conflict between animal and human rights, the human prevails.
So the central question is when does the being transcend from merely being an animal into a human, I think that the most important feature of a human that distinguishes us from animals is consciousness, so when a fetus first gains aspects of consciousness is 5 months post conception. So you could given that view that all abortions are valid. Also the argument of post-conception killing is irrelevant, as post-conception there is no conflict so there is no rational on killing/infringing on the babies rights.
I personally don't support this fully, as I believe that humans life for the future and for their future experiences and decisions, so I believe the standard should be likely to become conscious, given the technology that is developing this would mean that the window for abortions given my standard would become narrower and narrower as our society progresses. Basically I think that it is justifiable to deny the women a smaller set of decisions to allow the potential human to be allowed a larger set of decisions.
In the case of a danger to the mother, obviously the right of the mother would be larger that the fetus and can be justified using self-defense and in some circumstances lesser evil necessity. For rape and incest, I think that in cases were mental illness may be caused or significant mental harm then self-defense can be raised, but most women won't qualify, which is unfortunate for the women but we do have to remember there is a being that most likely will have future experiences.
The argument that the child will have a bad life is weird and some would say insulting. Ask a kid in an orphanage whether they would rather not be alive than alive. I think that they would say they would prefer to be alive. The premise of your argument only works if a significant majority experience such a life that they would rather not be alive. Otherwise there is no empirical evidence to support such an argument.