The so-called Pope's personal belief that he teaches the faith of Rome still doesn't change the objective fact of his heresies and apostasy. You're either less intelligent than you make yourself out to be, or, you know this and are of such a bad will that you don't care because it does not fit your heresy that heretics are part of the Church.
You are wrong on both accounts. I will attempt to explain....
My presumption is that since you quoted Pope Pius XII, Mystici Corporis Christi (# 23), you agree that:
Heresy is a mortal sin.
We know that all mortal sin severs from the Church, i.e adultery, murder, pride, and so on - but as PPXII said, due to it's nature, none sever like the mortal sin of heresy, (and apostacy and schism). Pope Pius XII is talking about the nature of the sin. Obviously you will agree the nature of the sin of heresy makes this sin the worst or among the worst.
The Church always calls upon all of her children to confess their mortal sins, do penance and amend their life.
For the Catholic who has fallen into mortal sin, (in this case the mortal sin of heresy), like all Catholics, he can receive the Sacrament of Penance and his mortal sin of heresy will be forgiven. Certainly the Catholic who fell into the mortal sin of heresy is in a much more serious spiritual condition than the Catholic who is in the state of lesser mortal sin, but neither is in the woeful condition of those who are outside the Church for the simple fact that one who is not a Catholic cannot receive the Sacraments.
I completely realize this truth is contrary to what sedes think, and in order to maintain your deep seated belief, you will need to deny the above truth - so let's hear it.
edit to add....I will be back later, real life is calling.