None can be *undone *or invalidated. They can all be *transgressed *in various ways.
A person who attempts baptism in another sect commits the sin of schism, heresy or apostacy, depending upon the circumstance. This does not “undo” their valid baptism. The sin they commit must be forgiven in Confession-- they must repudiate this invalid baptism and repent of their sin of schism, heresy, or apostacy.
Same thing. There is no “second confirmation” and it does not “undo” their valid, Catholic Confirmation-- this is merely the sin of schism, heresy, or apostacy. If they repudiate this and go to Confession they are forgiven.
Not quite. A priest who has not received a dispensation from the Vatican to marry, marries invalidly and cannot be reconciled to the Church until and unless they are properly dispensed or cease living in the sin of adultery.
If one is validly married, that Sacrament cannot be broken either. That is why the “remarriage” is not a marriage at all. A person who “remarries” is NOT married to this second person. Just like a “second” baptism is merely a farse, a simulation of the real thing, so too is a “remarriage”.
A person in such a situation is committing adultery.
They too can be welcomed back** if they cease living in adultery **or if their first marriage is determined to have been invalid meaning there was no Sacrament at all.