Hello! An update:
It's been almost a week and the despair I was feeling on Monday has lessened. Wanda is still in the med bay in the house. She still hasn't laid a normal egg without help. The shells of her eggs are getting thinner and more deformed, while the size of her eggs keeps getting larger. I feel like both of those things mean that laying is not going to be getting any easier for her... But I'm hopeful. She's still in good spirits otherwise.
She was working on an egg yesterday and I left her for a while to see if she could work it out on her own. By the time I was heading to bed it had been several hours and I didn't want to leave her all night, so I gave her a bath. She laid the egg in the bath only a few minutes in. By morning, she had laid another egg but this one had no shell. This is the first time she's ever laid an egg with no shell.
I have still been giving her calcium citrate daily, and offering her some egg shells which she's not eating. Should I start giving her two tablets instead of one?
The mass that is protruding from her vent that I've mentioned before - I'm starting to wonder if that is her uterus? Is that possible? Could it be inflamed and causing all these issues? I still haven't been able to source treatment (medication) so I'm not sure where to go from here. She is still having some yellowish discharge coming from her vent, but it's slowing. She is still eating, drinking, walking around, digging in my plants and having normal poops in between eggs. I added a dish of grit for her several days ago (I forgot that she needed that while she's been inside!) and she's been eating that.
On a perhaps related note, I mentioned that we may have Marek's in our coop. We've lost 3 chicks total now and another is starting to fade. The first chick died in May at about 8 weeks of age, and none of the other 11 chicks from her cohort have had any issues. We recently lost two from another hatch that were about 11 weeks old. The fading chick is from the same hatch. They've all had similar symptoms but not exactly the same - 2 of them have had leg paralysis, one has had wry neck, all have stopped eating and drinking, 2 died on their own and one was culled. At first I thought nutrient deficiency, but no treatment has helped any of them.
I also found two thin-shelled eggs in the coop today that were cracked, which is abnormal.
We also have lice, which we are working on treating. We've disinfected and sprayed the coop with permethrin, and my husband was able to find some spray today that we can spray on the birds, which we will do tonight.
Is there a disease that can cause these symptoms and also egg shell issues? Something treatable?
I'm so sorry this is such a long post, I tried to be concise

while also adding all the important details.