UPDATE #1: It's been 2 days since she got sick. No improvement at all, but she hasn't worsened or shown any new symptoms. Up until this morning we had been leaving her alone in the coop except to occasionally pick her up to inspect her for new symptoms and make sure the others weren't picking on her. I didn't want to stress her out and make it worse. But she seemed weaker this morning (from not eating?) and last night we noticed that she wasn't roosting with the others. Instead she was hanging out by the water feeder on the opposite side, acting restless in the dark. So I decided to isolate her in a dog carrier and bring her inside where it's warm. (She's currently in her little hospital sitting on my kitchen table I KNOW I KNOW IT'S GROSS watching me type this.) It's only been a few hours, but she seems less puffed-up/hunched and she has been drinking a TON of water (which can be a sign of toxins) and taking a few bites of soaked layer pellets! And she pooped! Although she's still mostly sleeping, fingers crossed eating/drinking/pooping are good signs and she just needs warmth and isolation to recover.
After exhaustively searching the internet feeling guilty as hell and hoping to find a more obvious (and not my fault!) explanation for her sickness, I am still convinced her symptoms and the timing align with avocado toxicity. While there are certainly many people out there with anecdotal evidence that avocado flesh is ok for their chickens, or chickens in Mexico, etc., the scientific literature I found links persin toxicity in birds with all parts of the avocado - it just depends on the size of the bird and their sensitivity to persin, the amount they ate, and the degree of persin leaching from the skin/pit to the flesh. DON'T FEED CHICKENS AVOCADOS! It's not worth the risk.