I agree on the gamebird. I would also let the mill know that they should not be recommending laying food to birds who are 2-3 months shy of laying. /sigh
Check for mite again several times this week - at night. Mites don't always stay on the birds, but only go back for feeding. The rest of the time they hide in the cracks of the buildings and even in the bedding around the birds. They only return to the birds when they feed.
If your birds are free-ranging and particularly if they have access to earthworms, or ground where adults were, I would consider worming them. I would use a true wormer, then consider cayenne and other products in-between. At this age, they could have ivermectin, fenbendazole, or levamisole. If they were older I'd recommend wazine first and then one of those other wormers in 2 weeks.
I personally use Ivermectin pour-on for my birds. Post here if you'd like information on that. Additinally, ivermectin will kill mites as mites suck the blood. It won't kill the ones in the coop, but if you treat the coop as IF there were mites you'll help prevent big problems with them in the future.
let us know if you need help with that two.
Use the natural stuff inbetween. Don't use it to treat an infestation if you know there's one.
As for buttermilk, you could alternately use a probiotic like plain yogurt - which still has the bacteria in it. The bacteria are the same that colonize the gut of your bird. Adding more beneficial bacteria occassionally will cause the birds to absorb more of their food, use more of their food. Using it weekly (or another probiotic like Probios) is recommended for birds under laying age, through the first llaying period, and any time a bird is underweight, stressed, or medicated. If you medicate, don't use yogurt (use Probios or acidophilis instead) if you're using a med that contains -mycin or -cycline.
Make the treats count.
Make sure also that the birds eat at least 95% of their daily intake in the complete feed. You could just supplement the protein with cooked eggs without messing up their vitamin/mineral balance of the feed you had. Consider returning the other food since it won't store for 3 months, or give it to a friend who has layers.
Or use as a treat - part of the 5%.
By the way, if they free range, make sure no matter what your ground is like that you also offer them granite grit. It will stay whole longer and is the right size for poultry. If you do that, you could offer some whole oats (after you worm) to help them gain body mass and condition without fat. The hulls of the whole oats encourage the natural beneficial bacteria in them to thrive. Thriving beneficial bacteria help ward off digestive problems and, again, nourish the birds. The higher protein in the whole oats is much preferable to scratch grains and corn. The wholeness of it encourages more nutrition to stay intact versus cracked corn, or the "treat" scratch. Again if that's offered, it should be less than 5% of the total diet. Just a little bit per day. Feed all treats in the afternoon, and scatter them so that the birds have to work for them.