Correct. The Pope can only speak infallibly on the subject of faith and morals. Therefore, when the Pope says abortion is a mortal sin, it is a mortal sin whether or not someone in their conscience can justify it.
Has the pope spoken infallibly on abortion? I was under the impression that he hadn't.
But assuming the teaching on abortion is considered infallible, if the pope's infallibility is only valid in areas having to do with faith and morals, then can't a catholic take a pro-choice position (which is a political position concerning the level of legal restriction on the activity) and at the same time adhere to the church's teaching by reserving a personal moral opposition to the act of abortion? After all, I don't think that very many catholics wish to turn every last moral teaching in the catechism into secular law.