I agree with alot of this.
I like HP alot - it's about escapism for me. I love film and literature and I love the subtexts of HP... about loyalty and integrity and living your life with honour.
I do not 'get' football. or any sports (except for Three day eventing and Dressage) and I find the concept of grown adult humans dressing in football colours and screaming / crying/getting drunk- identifying hard with a team to be totally alien to me. But I see that this is quite common and people all over the world want to identify with a community that speaks to them and that has shared values. That's normal and healthy.
It's no different to loving HP or going to comic con or joining a knitting circle or running group.
Miriam Margolyes is, IMO, a rather unpleasant person who says unpleasant things. This is apparently her USP and she and some people think it's funny. But given a good portion of her current fame has come from appearing in HP films....being bitchy about the very people who enjoy it seems a bit - well- pathetic.