Your concept of not being a person till you pass a magic line doesn't make much sense. Which conjoined twin is a human, & which isn't. Simply declaring irrelevance isn't sufficient.
Birth is a very objective line. The issue of personhood is irrelevant because it changes nothing. Sure, call a fetus a person, but abortion is still fine.
You are arguing circles. A baby would not die if it was removed from the circulatory system the day before it would be born. Therefore, you just declared it a human.
Late term abortions are rare, and usually only happen due to medical complications. They are irrelevant as they would happen even if regular abortions were illegal.
That is not a contradiction at all. Abortions happen during the first trimester, except in extreme situations. Late term abortions are irrelevant to the issue of abortion rights.