1. Not al phoetuses are viable even inside the womb. That is why miscarriages happen.

2. People don't have a right to use another person's body, regardless of need. So why give phoetuses rights that people doesn't have?

3. Someone born is independent. Anyone can take care of the born person and doesn't require to use other person body parts. That is, you can give a baby for adoption. If you give a phoetus for adoption, it might not survive even with incubators. So the phoetus that won't survive with an incubator, is not independant. While a baby that can be breast feed or formula feed by anyone is independant.

4. Siamese twins might share body parts, but are not the same as a phoetus, since the womb is not "shared" it belongs to the woman. It is interesting that you think that a woman doesn't complately own her own womb. Or at least that is what you "siamese" example implies. By the way, sometimes siameses are separated even if that means one of them dying. And many times both die if not separated. Sometimes the body parts are just enough to sustain one.

5. An unwanted phoetus that can't survive outside the womb is not viable by definition. After all, it would be removed from the womb and once outside, it won't survive. Force to carry a pregnancy because the phoetus might survive if the woman is forced to carry it is removing human rights from the woman. But as your example of the siameses implies, maybe you don't think that woman should own their own bodies.