1) Confirming the brethren (clarifying the Church’s teaching) is a fundamental role of the successor to Peter. If “he doesn't need to” means it is not his job, then you are in error.

If “he doesn't need to” supposes there is no bona-fide question, the numerous articles in every Catholic media about conflicting interpretations that are diametrically opposed and mutually exclusive contradict you.

2) While there may be a variety of opinions on a matter of doctrine, once the matter is decided these less-than-magisterial opinions count for naught. That one cannot validly remarry without an annulment is a settled matter, which renders putative remarriages of that kind sinful.

Doctrinal development builds, not reverses. That is the principle of non-contradiction.