They are at good ages and should be able to recover. I'm glad you have been able to separate them from the others. Vent gleet isn't usually transmissible, but if there's something stressing the flock, it can definitely spread.
The mites might have been a final tipping point for them. There are a few ways to deal with the mites, but the better ones, at least the ones avoiding lots of harsh chemicals or medications, involve you cleaning lots of stuff..
Mites tend to live in cracks on the perches where they roost. They will also dig into cracks anywhere the
chickens spend a lot of time. If the flock has mites, they will need treatment, too, or they will be a continuing problem.
DE works, but it takes time. Giving the roosts and any flat surface where your birds spend time a good scrubbing with hot, soapy water, maybe a coat of lime wash (the paint), and/or painting the roosts or flat surfaces will go a long way to helping get rid of mites and lice. You want to get the insects out of their hiding places, and seal off the hiding places. The hot soapy water will flush them out, the paint or lime wash will seal the cracks.
Then the DE can do its magic!
Yes, they should be showing improvement by now. I have read where some stubborn cases need frequent washing - I had one book actually recommend a bath once a day to keep the birds clean and dry - I thought that was too much, or for a particularly advanced case. You shouldn't need more than a couple of baths, maybe every couple of days but ... it depends on how severely sick they are.
They are stressed. They don't feel good, they can't be with their friends and family, and there's this person giving them baths! They will adjust to it. It's not as stressful as not taking care of them. I wouldn't worry about what you're doing stressing them. It's stressful, yes. It's also bad for them to not have the treatment.
Since they aren't responding to the Epsom soaks, you will need to go a little further.
Wash the chickens in warm water, with a little
soap. Carefully clean their vent area and remove any feathers that might be in the irritated area. They won't like it. Make sure you rinse them well with warm water to remove any soap. This also helps with removing any mites that might be on them.
You can dry them gently with a towel and let them air dry someplace they won't get chilled.
If you have iodine or Betadine available, or a similar supportive wound care solution, dilute that in water (iodine should be diluted, Betadine is usually used as a wash) and use it to rinse the area.
If you don't have iodine available, you can use ACV as a rinse to flush the vent and lower the pH of the cloaca. If the skin is very irritated, the chicken may try to get away.
Using ACV, or a dilute citric acid solution to flush the vent will help to lower the pH and remove whatever is irritating the vent. It's really important to remove the source of the irritation, whatever it might be, or they won't get well.
The Epsom salt soak is providing supportive care, but it isn't helping them as much as it should. That's why I'm suggesting a more thorough wash and flush. If there's a way you can get the ACV into the vent, gently, to flush the infection out a bit, that would really help them.
Please let us know how things are going for them. They should be showing signs of getting better. I hope they show signs of improvement soon.
This website recommends using antifungal and antibacterial medications, but otherwise offers good advice :
Vent Gleet treatment