“According to many there are other instances where abortion would be mutar besides for a risk to the life of the mother (e.g. mamzeirus, debilitating illness/disabilities).”
I am far from an expert in the halachos of abortion, but my impression from what I’ve always heard was that it is only allowed when there is risk to the mother’s life, and that risk to the mother’s life can involve emotional health as well. In the cases you mentioned, if it’s allowed, it is because the mother’s emotional health is at stake.
I read a nice story in one of Rabbi Dr. Avraham Twersky’s books. He had a patient for whom it would have been dangerous to her emotional well-being to have a child, so she had something inserted to prevent conception (I was a bit lost on the technical details). Something went wrong, and she ended up conceiving. Ending the pregnancy in this case would have been a much simpler procedure than an abortion.
So he asked Rav Moshe Feinstein, zatsal, if it would be mutar for her to do this.
Rav Moshe Feinstein asked, “If she had household help, would she able to manage emotionally?”
Rabbi Dr. Twersky responded, “Yes, but she can’t afford it.”
So Rav Moshe responded, “In that case, the problem is not an emotional health issue; it is a financial issue. Go out and raise money for household help for her.”