I have 2 unbreakable broodies right now, and 5 more I broke by moving them to a totally different environment. The 2 are hatching eggs for me today, and I'll let them raise those. I have one who's nearly done raising hers, too.
My broodiest birds have been bantam Cochins and Chanteclers, with a Barred Rock close behind.
I hope your necropsy comes back without any pathogens- that's a hard worry to have for too long, but I did send one girl off a couple of years ago and she came back as having had an egg broken internally and it had gone necrotic.
I hope you get good results!
I'm half-dead with bronchitis and no energy to even get out of bed. Hope everyone enjoys the lovely day!