I'd give it a few days longer, at least. I don't know what the timetable is, especially since we aren't sure of the cause, but I've been impressed with their abilities to recover, at least partly, from a variety of problems. Some have surprised me by living years longer, perhaps mildly...
I don't know what is causing this, but I thought I'd throw in that I strongly doubt it's just leg injury or bumblefoot. I have had several hens with sprained legs and several cases of bumblefoot, and their legs don't shake and they don't have trouble maintening balance as you describe. My guess...