In Catholicism, under what circumstances is 'abortion' condoned?

So... the Christian / Pro-Life stance is that terminating a pregnancy ends a human life. Thus, if done deliberately, that's homicide. Period. End of discussion.

Where things get nuanced is that homicide is sometimes justified. Governments, for example, have the power of the sword, which permits them to make war or impose a death penalty for certain crimes. Individuals may find themselves in a situation where they must choose between homicide and some other (possibly greater) undesirable outcome. The general term for the latter is trolley problem.

Abortion in the case of ectopic pregnancy is generally accepted because allowing the unborn person to continue to develop would likely result in both mother and child dying. Ideally, we would remove the unborn child to develop in an artificial environment (or possibly be transplanted to a surrogate). Lacking the ability to do so, however, the choice between the child dying and the child and mother dying isn't difficult; we save what life we can and chalk the rest up as an unfortunate consequence of the fallen and broken world in which we live. It's important, however, that the goal is to save lives; we aren't setting out to kill someone, but we're making a hard choice between saving one life at the expense of another or allowing both to die.

The other examples given are very similar problems; the mother's life (and therefore, necessarily the child's as well) is presumably in danger. If saving the mother's life necessitates ending the child's, especially if the child is most likely doomed either way, well... we live in a world where not all outcomes are as we'd like, but we still have a responsibility to do what we can. If, on the other hand, we're killing a child, made in the image of God, because it's inconvenient, that's murder. (And, yes, causing the mother distress falls into this category. It's literally no different from killing a senile family member because they're an emotional burden.)

Abortion that is the intention killing of a fetus in utero is a species of murder and hence intrinsically evil, thus can never be tolerated nor permitted morally.

The treatment of an ectopic pregnancy is not an abortion and is not considered one under Catholic morality. The intention is not to kill the fetus and hence it is not abortion, a species of murder. For this reason, it cannot be called "justifiable abortion" on parallel with "justifiable homicide" since the latter term describes cases where there is a degree of intention in the killing but where the law allows it (e.g. capital punishment, self-defense, etc). However, Catholic moral law would not generally call those homicide.

Murder being a grave intrinsic evil it cannot be permitted so the circumstances of the pregnancy do not justify the abortion.

Treating an ectopic pregnancy is an indirect (not direct) abortion.

Canon law defines abortion as the "ejection (killing+removal) of a pre-viable (immature) fetus" (eiectio fetus immaturi). (Killing a post-viable fetus (craniotomy) is considered homicide, not abortion.)

It seems like the question is more about ectopic pregnancies than anything else. You've already found the answer to whether an abortion is acceptable in case of rape or severe illness:

In Catholic morality, abortion is always wrong. Another principle is that intention is primary: it's about saving lives. When in saving the mother's life and reasonable care has been done for the unborn baby but the baby still does not survive, it's not called abortion as it is the unintended side effect.

I'd like point you to the Catholic/Natural Law doctrine of double effect. The direct killing of the baby in a pregnancy is not allowed as a means to a good end, but it can be permitted that the baby die as a side effect of another, morally neutral procedure (eg cancer treatments) if and only if the good end of the morally neutral procedure and its bad side effects are proportionate (in this case, life for life).

The doctrine of double effect applies to ectopic pregnancies. You cannot kill the baby as a means to saving the mother's life, but you can do surgery to remove the defective organ (the tube in which the baby implanted) or repair it (by opening it, removing the baby, and stitching it back up, if that's medically possible). You cannot do something like poison the baby with Mifepristone and Misoprostol because that makes murder a means to the end of saving the mother's life. This is not a "justified homicide," it is a murder, because the only justified homicides require that the person killed be guilty of grave personal sin. Since ectopic pregnancies tend to cause fallopian scarring and blockage even with direct (Mife. & Miso.) abortions, it seems to me personally the better option is the removal of the defective tube, to prevent future complications.

Further reading on Catholic opinions on the use of different methods for treating ectopic pregnancies: https://www.ncbcenter.org/resources-and-statements-cms/summary-ectopic-pregnancy