I'm not sure I completely understand this debate, as I haven't read every post, but formal repentance is necessary even if materially the damage can't be undone. If they have the intention to repent, they can be baptized, go to Confession, etc.
It's similar to any other sin that requires restitution of some kind. Let's say I steal $10,000 and spent it. I repent and intend to pay it back, even though I can't right now and may never be able to. I can be forgiven as long as I intend to do my best to make restitution. Then you have to try.
As for public vs. private Baptism, Baptism is inherently public, as it determines membership in the Church, and requires witnesses precisely for that reason. Yet, it could be quasi-private in the sense that the ceremony need not be performed in front of the broader public. This may be done to avoid scandal.