Luther was a Catholic?! Wow! The fruit of heresy is insanity!
That's right, not just a Catholic, a Catholic priest no less!
I presume what the canonist meant is that by Divine Law heretics are outside the Mystical Body, therefore the head of the Mystical Body could not be a non-member. The main point here is you think heretics are members of the Church. You reject one of the four marks of the Church, Unity. You need to read Satis Cognitum.
Do you realize that by calling Luther a "Catholic priest", you are saying he professes the true Faith?
Divine Law is law given to us directly from God Himself hence the name, "Divine Law", no one else is Divine, only God. Think 10 commandments. Hopefully you will remember this from now on.
In calling Luther a Catholic Priest, I am not saying he professed the true faith - the man was an abominable heretic. What I am doing is stating a fact of faith in virtue of the indelible character that the sacrament of holy orders imprints on the soul of all priests - and only on the souls of all priests. Even now that character remains - and will forever remain, marking him as having been a Catholic priest forever, which only adds to his suffering.
This is something 6 year old Catholic children are taught as part of their catechism, not sure what there is about it you cannot accept.