Thank you for this thoughtful comment! I really enjoyed thinking about the questions you raise.
You're right; this is definitely an ethical gray area, but I'm not sure I agree that Neville should tell Minerva if she doesn't remember on her own. If he did, it would be a way of unbalancing things, of gaining a sort of unfair power over her -- because the events still would not be things she remembered; they would just be things someone else told her, from their perspective; she'd have no way of judging for herself. I'm not sure what he would gain from telling her (unless just absolution, which wouldn't be fair of him to ask from her, I don't think), or she from knowing.
Still, she might indeed prefer to know. As you say, a gray area.
Thanks again for the comment. Here's one of the things I like best about Beholder and fandom -- the change to analyze the characters like this.