Imagine a person was abused for a large part of their childhood and is subsequently traumatised and mentally ill, then, upon regaining greater functioning as an adult decides to extort their abusive parents for money with the threat of exposing them while still counting on inheritence, instead of simply going to the authorities and approaching a legal settlement (expecting that will cut of any pleasant relations). Are there actions unconscionable? What would you do in their situation?

Depends on what you mean by "abuse"? A lot of what's been called "child abuse", e.g., spanking, isn't. On the other hand, legitimate abuse happens as well.