"do they really think that “repentance” actually wipes the slate clean, and/or immunizes against backsliding?"
How could they believe in "repentance" for something they don't think is a "sin"? Their actions show they don't think it is a sin. That's why they don't apologize to victims and that's why they hide their records. They haven't "repented". Why would they expect the other abusers to "repent"? They don't know what it means.
The only "sin" they believe in is the "sin" of speaking out against their WTBT$/GB/FD$/Bethel/Elderworship authority when they are wrong. (Which isn't really a sin at all, but rather is evidence of a person who keeps integrity...)
That is why God will give them (WTBT$/GB/FD$/Bethel/Elders) all into the hands of the secular authorities shortly and all their stuff (studios, real estate, prominence, etc) will be taken away.
Whatever secular legal group decides to take the lead in attacking the WTBT$ will be successful, in the same way Cyrus was. Jehovah will leave open the doors for them.