Conversely, why should the 99.99% of abortions that DON'T threaten the life of the mother remain legal for the 0.01% that do? (For the sake of discussion, I accept your premise that, except for ectopic pregnancies, artificial termination and PBA are even necessary to save the mother's life).
That was not the question. The question was whether someone who is against abortion to save the life of the mother could conceivable claim moral authority when it comes to respect for human life. And we were talking about regular abortions, not PBA.
The truth of the matter is that about 4% of abortions are in cases of rape, incest and for the life of the mother. That may not sound much, but it means that 40,000 women are victimized by these circumstances. And as long as you refuse to recognize that, or recognize it but claim that you know better what is good for these women (i.e., forcing them to continue their pregnancy), you will be unable to do anything about the 96% of the abortions that are done for the sake of convenience. Even the people of South Dakota voted against such a draconian measure by 10 percentage points, God bless them.