Latest on the girls' vent gleet:
They have ACV water and I'm sprinkling acidophilus into their feeder a few times a day. They knock a lot of feed out onto the ground, and I sprinkle more in when I see it's gone. I spend all day at home, I work from home, so I am able to check on their food often! I sprinkle 3-5 capsules on throughout the day.. more, less?
Bathing:
I bathed the two poopybutt chickens on Sunday & Monday, and they looked SO good by Tuesday that we skipped a bath day!

They were sufficiently poopy again today, so Penny and Lightning had baths. They are getting much better about the whole thing.
I liked what you said Chooks4Life about the "near death experience" and then seeing me as their "rescuer." I try to make it the least scary experience I can, I lower them very slowly into the water, I coo and talk to them, I pet them and you won't believe it.. today Lightning just fell asleep in my hand, in the bath?! HAHA! She went to sleep! She was on her side, held by my one hand, while my other hand cleaned her butt under the water.
I'm hoping tomorrow their butts will still be clean and we can skip a day, then bathe as needed (if they get poopy again).
Also, I have noticed a SIGNIFICANT improvement in the feathers around their rears! Where Lightning had no feathers, zero, zilch, she is now growing in feathers with the smallest puffs of down sticking out of them on her belly/butt.

Penny looks like a different chicken! Her butt is now properly fluffed, no more caking, no more drippies. They are definitely getting better
Lightning continues to be the most affected by vent gleet with the most poop stuck on her. She has normal poops, but what is stuck on her body is always white color. Also, I spent some quality time with non-affected chickens and I take back what I said about Lightning smelling "like a normal chicken..." She stinks, let's be honest.
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Chooks4Life -
Thank you, thank you, THANK YOU!!! Did I mention thank you?

No seriously. I've read and re-read your post multiple times over the past 2 days and haven't had a chance to respond yet. Thank you so much for going into so much detail, you explain things so well and really helped to put my mind at ease on certain issues!!
After your post, I decided it is time to work on taming the chickens I haven't been holding. I am noticing a marked difference in behavior with the chickens I have been bathing, so now I feel compelled to spend time with the non-affected chickens so they get to know me in the same way. Basically, if there isn't a reason for me to pick them up, I haven't been. Until yesterday, I never held Cersi or Rick Ross, but determined I brought them inside and sat in the bathroom with them.
Cersi.. oh man. Well she was freaked out at first, so I let her explore and feel "free" and what did she do.. she flew right into the bath water from the previous chickens. HAHA!! SO, she took a bath too!! Completely unnecessary, but perhaps made her feel that I was her "rescuer" by going through that ordeal? I have no idea.
After she flew into the soapy water, I dipped her in the rinse off bucket, then gave her a nice blow dry. She seemed to really enjoy it, I was so surprised! By the end of our session, she was happily sleeping in my arms, standing/laying on my hands (skin on skin like you said) and making the cutest sound.. it was like a cat purring? I just about died!! It was the cutest thing

I couldn't believe she tolerated me so well. It really gave me hope. I thought she was mean, mean, mean.
I hung out with Rick Ross indoors too, but I couldn't accomplish the same type of calmness/bonding out of her. she didn't have a bath, we just hung out. I'll try again tomorrow
I'm going to keep working on socializing them individually in this way. I bring a single chicken inside the house and hang out with them in the bathroom (since it's already covered in feathers and poop). I feed them treats and pet them, I let them walk around and I practice picking them up (they always freak out, but I hope that eventually they will learn that picking up doesn't mean you're going to die). I've discovered with Cersi and Lightning that they seem to feel safe while standing on my hand, only. They don't like the ground (even with towels down - not tile). Both of them were able to fall asleep in my hand, maybe eventually the rest will, too.
I am also working on having them walk away from me calmly like you said. I let them down onto the grass while their sisters are free ranging and they slowly/calmly walk over to them. Previously they were escaping me like a predator. Just from giving treats, some of the girls come right up to me when I'm in the grass with them.
My two cats have been doing really well near the chickens, also. They seem to know not to attack them, like you said, it's weird. The cats have gotten within 2 or 3 feet of the birds and there have been no incidents. The cat doesn't act aggressive or look ready to pounce, he actually takes a non-aggressive stance and squints lovingly at them. He must know they're ours and we intend to keep them.
Paleness -
I believe their combs and waddles have gotten a lot redder since we brought them home from the farmer. It almost seems that the largest (Cersi, Penny, Lightning) are the reddest and the smallest (Runty) is the palest. Also, I noticed that their combs would go pale when they fell asleep in my arms (Cersi and Lightning) while normally they are bright red, is that normal? I swear it's true. I can easily give them fresh garlic. In addition to ACV and acidophilus, they also get table scraps (green beans, corn kernels, corn cobs) and red and green cabbage, plus mealworm treats, grass, and any earthworms they dig up (we are on 8 acres).
edit: They're also primarily eating organic grain from their feeder inside the coop.
My SO is still considering wanting to add another few birds to the flock, while they're still young, and from the same original farmer/flock. I am not yet convinced that it's something we SHOULD or NEED to do. We will see...
Also.. After talking to my SO more, we came to the conclusion that the 4 Rhode Island Reds we got were the last 4 for a reason. We are obviously completely new to this and had/have no idea what we're doing. We went in knowing the breeds we wanted (brown egg layers, high producers, cold hardy) and that's what we left with. An experienced chicken owner probably could have seen that the last 4 reds were in sore shape, but we didn't. 2 were exceptionally small, and 2 were affected by vent gleet.
There were a lot of the other breeds, but we went in with a plan to get Reds and so we did. In hindsight, a larger and healthier Orpington or Plymouth Rock or Australorp might have been a wiser choice, but it's too late and here I am with my Red rejects

I love them just the same!
Edit: Just bought this, thank you for the recommendation!
The Complete Herbal Handbook for Farm and Stable by Juliette de Baïracli Levy