“Not everyone followed Rav Moshe in everything, mechitza, chalav yisrael…”
And on this issue, in practice it isn’t followed either.
“rape and incest is no more an excuse for abortion than if she became pregnant as a result of her immorality”
That is one poster’s (ok more than one poster’s) opinion. In practice EVERY such case I am aware of the Rav asked paskened an abortion should be done. Granted it is very very very rare (I can count on one hand) but unfortunalty rare doesn’t equal never.