Yes, it is true that prominent Mormon politicians have been pro-choice, as he himself was. But that doesn't make the church's position pro-choice. Just ask the Pope and Catholic bishops about that one.
You described a high level of flexibility in the official Mormon stance on abortion, I don’t think that the Catholic church does that does it?
Wouldn’t an openly pro-choce Catholic Bishop, raised by what Mitt claims was an early pro-abortion Senate candidate called his mother, be widely considered and known as being in rebellion from the leadership of his church?