last question — do rooster molt?
Roosters totally do molt.. and it could impact in exactly the way you are describing.. Molt is one of my top suspects since you say he IS eating AND drinking. You don't describe him as lethargic but more just quite and bit off compared to normal.. Overall it can be a challenging time for them. To give a real boost to the immune system Rooster Booster (brand) Poultry Cell (product) is superior to many others. I like to offer it on occasion. Mixed in a small snack is preferred over added to water as it doesn't mix well.
B complex would be (so much) better than B12, if possible.. thiamine, riboflavin, niacin, biotin, etc.. so many KEY B's that effect all kinds of functions in our bodies.. it's a good go to that does help boost the immune system!
Poultry Nutri drench is also a good supplement which includes amino acids that many vitamin supplements may not.
What are you feeding him on a REGULAR basis? My first birds molt when I fed "layer" feed saw many face this decrease in personality. Since switching to flock raiser (Purina).. the birds seem remain themselves more so, now going through less harsh molts and not withdrawing as in years past... Also calcium in "layer" is excess for birds not in lay and if fed long term *could* (
doesn't mean will) lead to gout or even kidney failure and sudden death.
Your lady in pics 1 & 2 post 5.. appears to need a saddle or a break from his fat butt.

(said with loving fun pokes at your boy) Her angry red skin is likely from him mounting to much BUT check after dark using a flashlight, part the feathers on the abdomen and below the vent and look for crawlie things running away. This is the easiest time to see them if they are present. Adding DE a drying agent is doing her already irritated skin a disservice. If you worry about mites or the like consider using something proven effective like a permethrin based product that requires NO egg withdrawal and is safe even for use on chicks and broody's while still being very affordable and easy to apply.
do you how much does a fecal test cost at a vet?
My vet is the highest price one in my desolate region and it runs me $26. Mail in ones are cheaper and some folks on here have used them. It can be done by any vet as the test is the same for cats and dogs.
Even if I say this or that might be better or what have you.. I'm glad that you're paying attention to your flock and know them well enough to notice something is off, you are looking for solutions and trying SOMETHING now, and doing your best instead of waiting until it's too late.
All birds loose weight during molt. Get a weight on him, figure out what his natural build is. He may be spending to much time watching the sky or mating as he is still kinda young.
Also, see pin feathers coming in or an explosion of them anywhere?