I don't get this as you had them on 21% protein feed and your vet dropped you down to a lower amt, yet, when there is feather picking and/or molting, higher protein often helps that.It usually gets very cold in the winter. We're between 20-40 right now (very unusual to be that warm). The coop has a insulated roof and we have two radiant warmers in there. It is comfortable most nights, but not warm. I'm just worried sick about the amount of feathers she keeps dropping each night. Does that sound like it could be feather picking? I wish it was a molt, but the vet said she didn't see new feathers coming in so she didn't think it was molting.
I've never used peepers, that's something some use to put on their bullies or feather-pickers which stops it. Maybe get a cheap camera or find a way to see if they or just one of them are picking her feathers.
Have you tried separating her out to see if she still loses feathers without them? Since your vet says no new feathers are growing in, then that would mean the chickens are plucking them or something is, as she should be growing feathers back.
I think your radiant heaters should be fine as long as she can cozy up to one if she gets cold. I have barely 2-month-old chicks out in a temp coop with weather similar to yours with one of those heaters and they sleep all lined up next to it.
