Demonstrate ethics? They'd have to be hired for that, I think.

Otherwise how do you demonstrate ethics to someone who doesn't know you, yet? :) Certainly not in a interview, that's just talk and smiles and promises.

I think the OP's 2nd and 3rd paragraphs are good demonstrations of their ethics. A track record of "doing good" and having lots of others vouch for you is a good way to demonstrate ethics to someone who doesn't know you yet.

Those paragraphs are not demonstrations of ethics, but of technical competence.

All you can do is give a good talk about why you're reformed. One thing that does work for you is that firms that decide to interview you must have given it some thought already, so there's got to be a chance.