I agree. But it has been the teaching of the Church that we cannot presume to know when another person is in a state of mortal sin. A confessor may be able to determine this, but not you or I. So when we hear that a person is divorced and remarried, we do not know the level of their consent, we do not know whether the conditions for a mortal sin were met.
This is not true. We know certain offenses against God and neighbor destroy sanctifying grace in the soul. We know a person who rapes a child commits mortal sin. What Jesus sys is we can not judge another person. We can not know if the rapist repented and has found mercy. If not the person is in the state of mortal sin.
Adultery and all sexual sins are mortal sins, says the Church. If I leave my lawful wife and run off with another woman I am committing adultery. The Church has always condemned this behavior. As long as I am living with the woman who is not my wife I am impenitent, living in the state of mortal sin.
Is there a difference between the person who looks at a woman lustfully and the person who has committed adultery multiple times? In one sense, yes. In another sense, no.
You are confused. The pope has no authority to change the constant teaching of the Church and his predecessors in office. Morality does not change. The pope can not say abortion is not a sin. He can not say adultery is not a sin. If any pope denies the constant teaching of the Church on matters of faith and morals he is apostate. Previous popes have addressed what to do if this occurs.
If some new matter arises that has never before existed and been addressed as far as it morality the Church may determine its morality or not. So if invitro fertilization is invented by modern science the Church can determine its morality. Or If cloning and genetic engineering begins to create designer babies it is something not previously addressed. The pope can not tell us that Mary was not a virgin, a matter of settled faith, or fornication is not sinful.
The Church can not say, oops, we goofed about xyz. Now we fixed it. Why? Jesus promised the Church would be led into all truth until the end of time. If the promise is true then we know what is moral and immoral with certainty. There can be no error ever. There can be no mistake to fix.