I have to disagree. The canon is not a minimum checklist for validity of the sacrament. It is simply what it says - an explanation of the obligations of any Christian under canon law. IMO, neither the inclusion of that canon in the code nor its wording affects requirements for validity.
You are correct that form, matter, and intent are required. In this case, your quibble is with proper matter. If we agree that sins are proper matter, then even before the OP updated her explanation in post #10, we knew she had confessed to leaving the Church. I question whether even you believe your implication that validity is lacking unless the penitent quantifies each and every type of sin. The confessor has wide latitude to truncate the details of a confession as circumstances require, and I fail to see a problem with it in this case.
Update: Now I see that while I composed this,
itsjustdave1988 repeatedly explained the same thing with more clarity than I mustered.
We’re not required to confess EVERY sin, only mortal sins, but we are required by Canon Law to confess those in kind and number. Not just one or two of them. Canon Law, while perhaps not the last word on sacramental validity, certainly doesn’t exist for no reason, and IS a binding obligation upon us! It may well be a fresh sin in itself to fail to adhere to its requirements.
Now justdave is referring to articles about FORGETTING to mention a sin, which is an entirely different matter altogether to deliberately omitting a sin, either because the priest doesn’t give you sufficient time prior to absolution or for any other reason.
Now whether matter is sufficient is something that is OBJECTIVELY determined. Not dependent on the subjective intent or actions of the priest. If he consecrates corn bread believing it wrongly to be wheat bread, the matter is still invalid and the sacrament is still invalid.
And if the sufficiency of matter is objectively determined, then those objective criteria that determine validity and sufficiency apply equally to all people concerned, surely.
In other words the criteria are such that a penitent can render a confession invalid by knowingly omitting a mortal sin. That being the case, surely that obligation exists, and the invalidity is present, whether the knowing omision occurs because of act of the priest or act of the penitent.
There aren’t two different sets of criteria, one for celebrant and another for recipient, in regard to any sacrament’s matter being valid. If validity requires confession of ALL known mortal sins, then surely it requires it under ALL circumstances, apart from the built-in exceptions under Canon Law such as general absolution.
Just as wheat bread, by way of analogy, is required for a valid Eucharist in ALL circumstances, and not just when it’s the laity supplying the bread as opposed to the priest making it with his own hands.