The way a law is written affects how it is followed. If a law is not binding, if a law has no penalty associated with it, then it is unenforceable. One is not obligated to follow an ambiguous, or penalty-less law.
For example, the 10 commandments have a penalty associated with them. They are non-negotiable unless you want to incur the penalty of hell.
On the other hand, there are "if-then" laws which one must only follow if they do a certain action. For example, IF one decides to go to communion, then one has to abide by the communion fast.
Lastly, there are laws which are ambiguous and which are unenforceable. For example, V2 is ambiguous and the Church has told us to "interpret" it according to Tradition (if that's even possible...but that's another issue). The fact that the pope has told us to interpret the docuмent means that it is, on its own merits, not clear enough to bind anyone to believe anything. A law which requires interpretation is not a law, according to St Thomas, for a law must be clear in order to be valid and enforceable.
You have this distorted conception that unless a Pope teaches "you must believe this under pain of sin", then you're free to take or leave anything in the Magisterium.
The way the magisterium teaches, affects the level of its authority. There is a difference between a teaching that is 'de fide' and one that is 'a certainty of faith' and one that is a 'pious opinion' and one that is 'theologically speculative'. Many times, in certain docuмents (i.e. an encyclical or a council) all 4 levels of teaching are mixed together. It is up to the Church to tell us the level of each teaching.
In the case of 'Amoris Laetitia' there are no roman officials who say that it is infallible. There is no one who says its teachings are 100% binding and have the authority which requires unconditional assent. Its teachings are a mix of theological speculation and ambiguous moral teachings. No one has to believe anything of this docuмent, no V2 official has stated it's binding and there is no penalty for ignoring it.
To ignore this docuмent is not to ignore Church authority because the topics covered are so basic as to already be officially taught in the catechism and don't need any further explanation.
He keeps coming up with this under "pain of mortal sin" thing; but has never explained where does he even get it from, even though he has been asked to do so.
You've never asked a specific question about this issue until now.
'De fide'/Doctrinal teachings must be believed under penalty of heresy (which is a sin). Non-de fide teachings are binding to the extent that the Church tells us the level of teaching authority. Each level of teaching authority has different levels of assent (and consequent penalties) associated with it.
A non-doctrinal law MUST require a penalty be named in order for enforcement because non-doctrinal matters fall under the govermental/human aspect of the Church, where the pope can "bind or loose" as he pleases. Therefore, in absense of a penalty, then such a human law is inconsequential.
Example: The pope creates a Confraternity of the Rosary or dedicates that a new Saint has a feast day. These are laws but they are optional because there is no penalty for ignoring them. No one has to join the rosary confraternity under pain of sin or under penalty of any kind. Therefore, in a sense, the confraternity is "optional". I'm not allowed to deny it's existence, just as I don't deny that V2 or Amoris Laetitia exist, but I'm not obligated to join, promote or support the confraternity or any of these docuмents.