Thank you so much as always, you are so helpful and so generous with your knowledge!I highlighted some questions and points in your post that I will address. I invite other OTs to chime in because this is just MY take on the situation.
Question #1. There are many ways to fatten up a chicken but I think the most important thing is to find out why they are not in good condition in the first place. Not correcting that problem and just adding more fats and proteins to the diet is just throwing money down the toilet. Any reasonably healthy bird should stay in good condition on free range and regular laying ration feed, no matter the actual fat content in the mix. Correct that original problem and they should just fatten up on what you are feeding...if the rest of the flock stay fat on it, why not this bird?
#2. Yes. Sometimes mites will be on some of the chickens but not all. This can eventually change the longer the mites are present in any chicken in your flock until they are all infested.
#3. Some high fat feed formulas will tell the recommended feeding ratio on the bag like Calf Manna Pro. Not sure, but I assume the rest are the same. You can add fats to their diet with regular cooking oils, suet, bacon grease, lard, etc. and just give it in small amounts at first and see how your flock tolerates it. The people who show can best advise you on how to put more fat and conditioning into chickens but it still won't help much if you don't get rid of the source of their problem.
Points in the post:
I'm not a fan of nor have ever used the DE, so cannot reliably discourse on the merits or lack thereof, but can tell you that plain old wood ashes dusted deeply into the skin of all my flock had most of the flock mite free within a few days, with a second application getting the remaining three still having them. I also combined this with applying NuStock on any mite eggs or lice nits at the base of the feathers around the vent.
Give the FF time to work some magic..they've only had it a week, so give it some time to get some probios into their systems and to change the pH and nature of the intestines and their function. Patience is needed when using more natural approaches to care...not like high powered shot or pill.
BOs, when fed plenty of feed, will prefer to eat if from the trough instead of going out and hunting for it. Wrong breed to expect great foraging instincts, though DragonLady has some breeder quality BOs that seem to have retained that natural instinct. If you really want them to forage they have to be a little more hungry....only feeding in the evenings will increase foraging desires, as BOs are used to having their tucker.The problem with trying to increase the fat and nutrients to the poorly BOs is that it will increase the weight gain on those who are already fluffy..and may lead to digestive or reproductive issues.![]()
I'd add the suet like you are doing, wait for the FF to correct some wrongs, dust with something more effective than DE, use some Nustock as needed and also get some ashes in the nest boxes, NS or neem oil on the roosts. Then start paying attention to coop and soil balance to prevent further infestations.
That's just my take on it....other OTs may present you with more options in treatment. Let us know how it all turns out and what methods you used and the results?

I know how you feel about the BOs

On another note, I'm considering trying some dominiques as they are supposed to be good hardy DP birds and very good on free range. Any OTs have experience with this breed??