I disagree with you that false piety is why Paul wrote. By quoting the way you did you have led a false impression. Let’s not get into a debate that takes us from the point which was that taking a vow of virginity was not unheard of even if as you say it was not uncommon. The point you have brought up does not dispute that statement.
I disagree with you also that when we agree it is what is plainly understood. I see the opposite when something is not plainly understood we agree. It cannot be plainly understood what Joseph and Mary knew.
As to Cana, Jesus tells Mary it is not his time. That is plain to me that Jesus believed Mary was asking Him to perform a miracle. As her only child, she must have known what He meant that it was not His time.
God gave no instructions to Mary or Joseph not to have relations after the birth , her first fruits. What declaration is there already from God to abstain or not ? Again , I am not assuming any vow the two had taken previously in this regard
Is everything written down what was told to Mary and Joseph. Obviously not as is demonstrated through the conversation at Cana. Mary’s vow to be a virgin would not have necessitated an instruction from God. Joseph being a just many would not have touched Mary. He would not have had to know full Christology all he needed to know is that the pregnancy was of God that would alone prevented him if Mary’s vows hadn’t of.