A couple of things.
AL is NOT ex cathedra, it does not IN ANY WAY touch on the issue of papal infallibility. It does, however, touch on some other key issues such as papal authority and the nature of objective truth and perhaps whether or not such a thing even exists.
As quoted above Lumen Gentium says:
"Religious submission of mind and will must be shown in a special way to the authentic magisterium of the Roman Pontiff, even when he is not speaking ex cathedra; that is, it must be shown in such a way that his supreme magisterium is acknowledged with reverence, the judgments made by him are sincerely adhered to, according to his manifest mind and will."
Is this statement of Vatican II true or false? Period. Stop trying to qualify it.
The fact that AL is not ex cathedra is IRRELEVANT if the above statement is true, it explicitly says so.
The pope has said that the authoritative interpretation of AL is the one given by Cardinal Schonborn who has himself said it is magisterial and part of sacred doctrine. If you want to say the pope and one of his chief theologians are wrong - on what do you base that? What are your qualifications to do so?
Look, the very people who are criticizing Pope Francis and Amoris Laetitia would be having an absolute aneurism if I said the exact same things about John Paul II and Familiaris Consortio. Don't deny it. What's the difference? Francis is pope. JP2 was pope. Amoris Laetitia is a post-synodal Apostolic Exhortation. Familiaris Consortio is a post-synodal Apostolic Exhortation. The only difference is the content. I don't like the content. I don't agree with it. So I'm going to dismiss it. Since when can a Catholic pick and choose what he or she will and will not accept? Again, if I said that 30 years ago about Familiaris Consortio, I would be labeled disobedient, a heretic, etc.
If you want to say I don't have to pay attention to AL because it's heretical and Francis is a heretic and not really pope, that's fine, make that argument. If you want to say Francis is the vicar of Christ, possessing the keys, so he can change moral truths and redefine things like adultery, that's fine too. But stop being inconsistent and hypocritical applying Catholic teaching one way when you like what the pope says and another way when you don't.