Adoption is a red herring in your point combined. Adoption is irrelevant in every point you make. Yours is a point regarding a toxic adult and parent relationship.
Nothing will happen against any abusive parent if the abuse is never reported. Adoption is irrelevant.
A grandparent would be considered (not necessarily granted) as a carer if a parent dies and there are no criminal charges against them. Adoption is irrelevant.
In the case of delayed reported abuse from an adult survivor, no parental responsibility exists because the person is an adult. Adoption is irrelevant.
A long-lost parent could assume parental responsibility later in a child's life (I am thinking an absent father arriving in a child's teenage years) and unreported abuse could causes the breakdown of that relationship. Adoption is irrelevant.
Combined, what makes you think this is an issue to do with adoption?
I am all for varied discussion from a variety of angles, including the unique opinion of an adult who was the product of an unsuccessful adoption. It adds new dimensions to discussions. Unless there is a huge amount of projecting happening here, I have no idea what the point is you are trying to make?