I am a newer owner (a little over a year) of personal snakes as well, and I've been fortunate to not have to deal with mites (yet?!?). I did a lot of research before I got my boys, and it sounds like you are doing your very best! I love this site, as I have gotten so much good information! I've been very afraid of mites, so I dry my Cypress and Aspen mulch in the oven before I put it in their enclosure just to be safe. Maybe overkill, but I have happy little boys!
It sounds like your little one came with them. What a bummer. You can do this!