Actually, a Catholic theological analysis of the life of the mother exception would be that the baby was unavoidably killed as a side-effect of saving the mother. That might be the case, for example, in an ectopic pregnancy.
Similarly, it would be illegitimate in traditional Catholic teaching for a married person to have an operation explicitly intended to render him or her sterile, in order to avoid having children while enjoying carefree sex; but it would be perfectly legitimate to have such an operation if necessary to remove a cancer, even though it had the further side effect of rendering the person sterile.
As another example, it would be wrong to remove your arm because you thought you looked better without it, but it would be legitimate to remove it because it was incurably infected with gangrene, so as to save your life.
Perhaps to some this sounds like hair-splitting, but I believe it's the right way to approach such questions.
Also, what you have described here is really common sense!