Everyone else has pretty much covered how you handled it. On the MoH point, there isn't a power differential or duty of care to her that makes it unethical in that sense. I think as long as you are open and clear about why you're asking her out - whether it's "I feel terrible and would like to buy you a drink as a friend to make up for it" or "I feel terrible, but man you are badass and I would love to go for a drink with you, would you be up for a date?", I think it's fine.

Also while I think you did fuck up, I think you're also being quite hard on yourself. If the genders were reversed and it had been a man hounding his ex while she worked it would have been viewed quite differently. You were being harassed and that's a crap situation.