I use this which you can apply topically or orally (depending on the need) via syringe dosed for a chicken by weight: https://www.farmandfleet.com/products/1263615-durvet-ivermectin-injection.html
I get mine from Meyer Hatchery they've all been pretty amazing. That's not to say there haven't been issues, and the two that I lost seemed to follow external forces/events, and a bit of self blame for not catching it quicker or doing things wrong.
Feeding them too much treats, and situational...
Just noticed your last post. I've had buckeyes look worse than her and just be going through a molt (I assume) I can't really say yes there is a problem just looking at those pictures, she doesn't look bad to me. I've had a few issues with Buckeyes over the years, but that could be inexperience...
I haven't used olive oil myself, but I believe I've heard of people making use of it, not exactly sure for crop issues. Have you checked out the articles on crop issues here? Shortening from my perspective seems like it would gum up, and it's definitely not good for our arteries, but that's any...
Try bringing another chicken in with her, and put down some straw so they can sort through it. Her mate might perk her up, I've seen it myself. I've been through this before a couple times. Be careful with inducing vomiting. I've not done that purposefully, but I have had a chicken choke to...