I recognized the ambiguity in the soul sparks line. The part about stillborns, which conversation with Rama did not concern the question you and I are debating, is what much later sparked an aha moment in me.

Dependent on the mother is not convincing for the one body argument. They each have an entire separate set of every human organ. Unlike the conjoined twins example I gave in my PS.

The fetus is merely connected to the placenta by the umbilical cord through which nutrients pass to the fetus. They don't even share blood.