If its redmite you should see them glow in the dark with a torch if you go out to the house at night. Pretty sure they probably aren't cause of red bums though unless they are dreadfully infected and they would be quite unwell by that stage. Do they perch at night, some chooks lay in the bedding and if its soiled that will not be good, as you will know from meat birds, they can get ammonia burns, which may cause this-if you have used powders that should solve any mite problem actually on the bird, but sore patches won't heal over night.
If it turns out to be redmite-don't hold your breathe over getting shot of them

. I flatly refuse to use Creosote which is the only sure fire option (on the house not the chickens that is). We resorted to buying new house, powdering birds for three days before using it, having a small fire in the run where the house previously stood, not putting new house in same spot. Also totally Poultry shielded house prior to use, then diatomed everywhere. Will muck out every three days retreat with poultry shield and diatome each time. It was such a problem with the hybrids last summer am now absolutely paranoid. Old house will be dismantled and left to see if can clear out well enough to use for waterfowl, if not will burn it.
Don't know if thats any help or just an added annoyance to your situation
