One can't usefully define 'rights' when you use such a moving target.
If ending the child's life depends on viability, or in your terms, sustaining its life without the mothers aid, then we have entered a very fuzzy gray area. First of all, what do you mean? Do you mean, when the baby is able to continue having its heart beat without the blood supply of its umbilical cord? Or do you mean that they are 'viable' the moment that they can supply themselves with food, say, somewhere around five years old? Your term viable leaves such a large amount of interpretation that it really constitutes no principle at all, but rather allows people to just define what is right and wrong entirely for themselves, and to end the life another human being based on their own subjective preference.
I understand you are simply addressing abortion, but your viability concept essentially sees the baby as a parasite until such time as it can fend for itself. The unintended consequences of this could be catastrophic in their later legal implications.
One does not build law (private, natural or public) on the basis of subjective preferences for the simple fact that then law means absolutely nothing.
If you have defined life itself as a subjective belief, then you have attacked the very roots of libertarianism itself. Without a sensible definition of life, how can anyone have right to life? How then can we deduce the NAP? How can we have property rights? All of these become subjective preference themselves to be discarded at the whim of whomever has the ability, or power, to do so. No legal or political principle then exists. You could protest, prior to your execution, that the executioners have no right to end your life because you have not ended anyone else's, but using your understanding of the concept of life, they could simply say that they do not believe in life they way you do.
"Is life so dear, or peace so sweet, as to be purchased at the price of chains and slavery? Forbid it, Almighty God! I know not what course others may take; but as for me, give me liberty or give me death!" - Patrick Henry
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