Hello: Are you sure R’ Elyashiv even holds like R’ Moshe. But assuming he does, and assuming that we still can’t call the baby a Rodeph because his existence is preventing the mother from taking a life-saving medication (which is a tremendous assumption), then the issue is not the “kill or be killed” issue. The mother wouldn’t be doing the killing, the doctor would. Thus, we would be looking at the Mishnah and Gemara in Horiyos 13a which lists who to save first. Now, the Gemara (or Rishonim) don’t discuss anything with a fetus, presumably because such a case was inconceivable where we could choose between saving the life of a fetus versus a person, especially when we take into account that the Mishnah says we kill a fetus to save the mother. So we have a choice between saving a woman who is Vaddai living and a Vaddai Kasher, versus saving a fetus who we can think of a limitless number of Sfeikos about. We should also save the woman because she can claim that the baby is a Mamzer and thus she would get preference either way.
And I don’t think you’re right about this not being a Grama. Without getting into biological details, the chemo does not directly attack the fetus. It will affect things inside the mother’s body that will eventually lead to the detriment of the fetus. Aside from this, chemo won’t definitely kill a fetus (there are documented cases of a fetus surviving chemo for as many as 5 months). Which might make this a case of Safek putting someone in danger to save someone else’s life, which is obviously not allowed. But once again, these two are linked so it is up to us to decide who to save first.
Also, even according to R’ Moshe we should call this fetus a Rodeph. Pregnancy and labor is always physically traumatic regardless of the health of the mother. For someone with a late-stage cancer, going through something that physically traumatic would always increase the possibility of life-threatening complications and it’s impossible to claim that there isn;t a Safek Pikuach Nefesh issue for the mother here. And unless I am mistaken, it is clearly brought down that you kill the fetus even if there is only a Safek on the mother’s life.