Thank you so much for sharing these pictures! It helps to see what the bald spots look like, and the cause for them. Being underweight could be caused by parasites, but are your Sex Links and Rhode Island Reds actually underweight, or do they appear to be smaller when compared to your other birds? I know that you mentioned that your previous SL was much larger, but it could be that the bird was overweight.
For example, the average weight of an RIR is around 6.5-7.5 pounds. I recall my own RIR looking pretty small in size, but she was pretty hefty in weight. Regarding their feather loss, it looks to me to be from feather picking/plucking, but, as
@Wyorp Rock mentioned, it's a good idea to check for mites or lice, just to be on the safe side.
Stress: Regarding stress, the birds might be feather-picking when you're away because they might be a little bored without being able to free-range. It doesn't sound like your daughter's way of caring for them is bothering them. Rather, I think they might resort to broody behavior or feather-picking (which could be why there are missing feathers), when there's a change in their usual routine and they can't freely roam. This could also explain why you don't see the chickens usually pecking each other, as they only do it when you're away and aren't free-ranging. (Keep in mind that this is just a theory, and overall, your chickens sound like they're living a very happy life!)
Protein: It sounds like you're giving them plenty of protein, and I'm so glad that they have access to water 24/7! I'm not sure how many treats you give them, aside from their weekly mashed eggs, eggshells, quinoa, and spinach. Besides this, how often do you give them the other treats? (e.g. The black fly larvae.) While this does provide protein, they may need a little less if you give it to them a lot, as they don't hold a ton of nutritional value.
Parasites: As I mentioned above, while the bald spots do look like feather picking and plucking, be sure to check them tonight to see if they have any mites or lice. I'd love to know what you find out!
While spots on the comb can indicate Fowl Pox, my understanding is that the spots are generally larger, wart-like bumps (with dry pox), and can sometimes have a whiteish hue to them. From what I saw in your pictures, the bumps looked like it was most likely caused by a couple of pecks from a fellow chicken in the flock. (My chicken Jasmine often gets similar-looking spots when she's pecked. However, it's a little hard to tell with the pictures you provided, as they weren't focused on the comb.) All that said, I don't know for sure, but I wouldn't worry too much about them. Please feel free to correct me if I'm wrong on that!
Thank you so much for sharing those pictures, and I hope we solve this soon!
