Fair points but:

1) Science lacks the ability, using it's own tools (e.g., the scientific method) to prove or disprove anyone's opinion on When Personhood Begins. You've provided a totally rational POV on the matter, but another person could, also totally rationally, point out that interrupting the cascading sequence of development, once that sequence has started, is just as murderous as interrupting the sequence after a later time. There's no resolving that argument.

2) I've always found the thought experiments with the "who do you save" question interesting, considering that in many states, if a pregnant women is attacked (or in a car accident) leading to the loss of the fetus, the perpetrator can often be charged with some variant of a murder charge.