Cutting a marriage because it’s no longer satisfying but without an intention to remarry is abandonment. When the Church says that what God put together man should not put asunder, this does not mean that everything is fine as long as one doesn’t remarry. This said, a civil divorce may sometimes be necessary, but I think in many cases it seems to be a final act of interrupting marital cohabitation for good, in which case it is not necessary and gives rise to wrong ideas, such as that a sacramental marriage ends, which it cannot, as we all know.
Masturbation is grave matter, but many masturbators suffer from defects of reason or will. Where’s room for a difference of opinion here - other than merely disagreeing whether it was already a mortal sin or not yet on the account of the actor’s knowledge and consent? As you say later in your post, both can be looking at culpability, however I’m not sure there’s much that can be said about “angles”.
One priest is right and the other is wrong. They cannot be right at the same time if their opinions contradict each other. Further, one act is either mortally sinful or not, but not both at the same time. We might not know which priest is right and which wrong, but objectively one of them is right and the other is wrong.
That only makes the difference between adultery and fornication. Say, the person was forced at gun point, but all witnesses lie and the tribunal is misled, issuing a decree for validity. The person then has sex with a third party. I don’t think anyone would call it adultery with the understanding that the forced party was really forced. However, fornication remains since that person is not married to the third party. That person is in a horrible situation, but still cannot just have a civil marriage and have sex with the third party.
Any person who has attempted marriage needs a decree. The marriage possesses the favour of the law. That person will not get a sacramental marriage. And if that person will not get a sacramental marriage, there will be no marriage. One cannot have sex with a person with whom one has only a civil marriage just because one is certain that his marriage is invalid. Even in an obvious case, as in my example of someone being forced at gun point and all witnesses lying, one still cannot decide that the person needs no decree and that civil marriage is now sufficient to avoid fornication. The fact that one is unable to marry does not mean that one can have non-marital sex without sinning at all. It will just be fornication and not adultery.
Even a justice of peace marriage requires a decree, although the trial is a simplified one in those cases. But this cannot be skipped. People should resolve every attempt at marriage before beginning to date.