Kit Kat, with respect, I disagree.

If you define biological separation in broadly general terms, then you might be able to define separation only as complete independence. Reductio ad absurdum, if you generalized it on a larger scale, we're all dependent on each other globally, so does that mean we aren't biologically separate? No. Separation doesn't imply absolute independence.

I think you supplied your own answer. "A fetus that depends on the woman for absolutely everything, which cannot, in fact, develop outside of the womb, which is physically connected to the woman and whose biological functions would cease if removed from the womb--arguably, it is not biologically separate." Dependence on another implies distinct existence. The logical antecedent of "depending on someone else" is that the dependent is not the same as the provider. Whatever the fetus is, it isn't the mother.

In the linked article to the maternal RNA, Saletan quotes Robert George and Christopher Tollefson as arguing that the mother is the developmental "tugboat" (my phrase, not Salaten's). Basically, like a tugboat, the mother pushes the fetus along until it gets out of the harbor and can sail on its own accord. Saletan counters that the mother isn't a simple tugboat, and that there is no hard and fast "hull" where tugboat ends and ship begins. My answer is that as long as there is any distinction between ship and tugboat, i.e., between fetus and mother, then the point is made.