Yikes, that sounds really stressful for you and Mango!
I don't really have any advice for you since this is not something I have witnessed before, but I would want to separate those two just for the sake of peace in the flock.
When I had an all-hen flock, the alpha hen mounted a few of the others, but not all of them. I couldn't figure out why she chose the ones she did as none of them were particularly pushy hens.
I have never seen her pull out feathers or harm the others in any way though, so it might be a bit...