Maybe I’m misreading your question, but I believe Canon Law 915 would protect a priest from administering Holy Communion to someone known to not be in a State of Sanctifying Grace.
But if the issue was something different, like say, a traditional-minded Catholic priest refusing to deliver a sermon wherein the priest was required to preach that the doctrine of Religious Liberty allowed any person to have the right to practice any false religion that they so chose, and he refused to do so. I believe in that case it would not be a mortal sin despite the fact that his decision would be in direct disobedience to Church doctrine as proclaimed by Vatican II.
Of course modern bishops would disagree with that opinion, but that is largely what the current debate is all about that is going on today between the SSPX and Rome. Obviously it involves more than simply the teaching of Religious Liberty, but all of the heretical doctrines proclaimed by the Second Vatican Council.
I have to wonder with the recognition in Argentina whether the SSPX will continue to fight the heresies or if they will become just another conciliar indult group.
But otherwise a major thumbs up for your post.