Your last point is a good one, I also wondered the same. Wouldn’t the added stress on a pregnancy on a sick woman be a rodef. I have to assume this was taken into account and we are not smarter than the Posek who ruled this way.
I’m not an oncologist, but are you certain the chemo would not kill the fetus directly?
Again, I did not say I would have ruled this way, but i don’t understand why you find it so inconceivable.