He likely doesn't have internal injuries, it just depends on how beat up he got. He probably just has a pecked butt from the other chickens and possibly from him trying to groom the mites/lice off.
The corid will kill good and bad bacteria alike, so after treatment be sure to give the chickens some probiotic supplements.
You can't get the mites, they can't live off of you. I would clean the coop out, put in fresh straw or whatever you use and dust the fresh straw with poultry dust. Then I would make a point to powder all your chickens. You can fill a sock or panty hose with the dust and use it like a lady's powder puff to dust the chickens. Get under their wings and around their vents. Then set up a place where they can dust bathe in the powder mixed in with some dirt.
Also if the powder you use is permethrin be aware that it is highly toxic to cats, so put it in an area that cats can't get too.
I know it seems overwhelming but you'll get it under control soon. *hugs
MA is too but ya. 
there are parts here that are pretty though. I've never been out west. I've only been to all of the New England states (so Massachusetts, Rhode Island, Connecticut, New Hampshire, Maine, and Vermont) although Connecticut I've only driven through to get to New York. Would like to actually visit. So been to all those and then I've been to New York and Pennsylvania but mostly just driven through NY to get to Erie as we have family in PA. And been to Florida. I've technically been to New Jersey and Georgia but only a layover so I dont count it. But anyways, we live here so it's easy/common to go to the other states and we have family the other 2 so that's necessity too. And Florida was just fun but I haven't been there or anywhere really in a few years. I'd like to see other parts of the country. 
