I stated public sin of manifest "formal" heresy.
I don't think so because your #1 and #2 merely state that the sin of heresy was being committed. Certainly all heresies are in direct contradiction to what the Church teaches, but for the sake of argument, your #s 1 and 2 were coming out of the mouth of an infidel, a prot minister, or a prot child for that matter. In these cases, I don't think we can say with certainty that they are speaking manifest heresy.
Instead of saying "One who knowingly...." You could have phrased it: "A Catholic who knowingly..." in this way it would have been more clear I think that the person was guilty of the sin of heresy.
Through Confession, "A" is reconciled with the Church. Given that "A" was a member of the Church before his public formal heresy, he has the knowledge about the Church's tenets. Therefore, there is no need to reinstruct him on the Church's tenets prior to administering the Sacraments again to him.
But all non-Catholics, all of those outside of the Church, all non-members cannot receive the sacraments. So if this Catholic guilty of the sin of heresy, per the OP is ipso facto outside of the Church, then he cannot simply go to confession and be absolved as if he is a member of the Church, again, this is per the OP.
In reality, manifest heresy is a mortal sin, for this particular sin the Church attaches a censure, the censure of excommunication. In confession, in the traditional formula for absolution, the priest first removes the censure, then absolves the sinner. This is true for all of us trads each time we go to confession.
And that's why a Catholic who has committed the sin of heresy is still a member because he can simply walk into the confession like only Catholics can do and leave absolved, whereas non-Catholics cannot.
I am granting you here that what you said about "A" is true. You have not provided evidence, however. If you deny that the public sin of manifest formal heresy makes a Catholic become a non-Catholic, then you deny the teaching of Pope Pius XII in Mystici Corporis, Paragraph 23 because that teaching is specifically about Catholics becoming severed from the Church via public heresy (and apostasy and schism).
The sin of adultery does not by its very nature make one a non-Catholic. It is by its very nature a mortal sin, but it does not by its very nature sever one from the Church.
PPXII was of course right. The excommunicated Catholic cannot partake with the community of the Church, he has excommunicated or severed himself from that due to his sin of public heresy.
This means those who are excommunicated are forbidden from taking part in the communal life of the Church, they cannot receive communion or any of the sacraments until their sin is forgiven, they cannot be a sponsor, they cannot sing in the choir, be an usher etc. etc,. The nature of the sin of heresy makes one position themselves in direct opposition to God, the Church, her teachings, her doctrines, her magisterium and all things Catholic. They have effectively severed themselves from the Church by their sin of heresy. But if they were ever Catholic, they are still a Catholic - guilty of the sin of public heresy - and need to go to confession, which again, is something only Catholics can do.