I hope your Buff is better this morning. Good for you for being so observant.
What would I do if she were mine?
You did a good job looking into her mouth. If it was something like gape worms you would of seen them.
Speaking of worms I do not know how old your hens are but if you’ve got serious worms you’ll notice weight loss even with a good appetite. Runny poops and more often than not you’ll not see worms in the poo if you do then it’s pretty serious. You need to be sure and pick a broad spectrum wormer and do a follow up dose in 10-12 days to kill any eggs.
You mentioned it’s been raining. I think wet=mold.
Perhaps inhaling mold spurs or pecking and eating something with mold.
Inhaling mold spurs can cause respiratory issues.
Ingested mold can cause crop issues.
It’s good you’re taking her to the vet. Be sure to mention all the environment conditions.
What would I do?
I’d start by putting ACV in their water. You need to use a glass or plastic container.
Apple Cider Vinegar is corrosive and will have a reaction to metal containers. It will ruin your waterers and open them up for alge and other bacteria to grow.
I mix a shot glass (1oz) into a gallon of water OR 1tsp. Per liter. Change it daily. I’d offer it to all the girls. ACV not only effects the Ph level of the crop it has vitamins & minerals and trace elements that help the immune system. I only use ACV if I suspect respiratory problems.
I would also give her some chilled coconut oil. Mine will eat it right off my fingers. It has healing properties. It could soothe her throat and it will help with her crop.
I tend to use simple things to help the chickens boost their own immune system.
Good luck. I hope all goes well. Keep us posted.