Sounds like your old gal is going through something. Since you’ve already given her fenbenzanole, maybe keep an eye on her hydration and try some extra vitamins in her water. If she’s still acting off, a vet visit might be a good idea to rule out anything serious. Hope she perks up soon!
Good call on the Miconazole. Yeah, it's best to skip eating her eggs while she's on the meds. Most treatments like that make the eggs unsafe for a bit. Better safe than sorry, right? Maybe give it a couple of weeks after treatment just to be sure.
Man, that's a tough situation with those broody hens. It's wild how they just won't break, even with all your experience. It could be a breed thing or maybe some odd selective breeding practices. Either way, it's definitely not the norm. Hopefully, someone out there has some insights for you...