I'm honestly thinking it's Marek's.
As far as the "let her go" question - I leave that up to the bird to tell me. Is she still eating and drinking? Do you feel it's time to let her go? Only you can answer that. If she's eating and drinking, I would weigh her in the morning (using a kitchen...
I would like to suggest a sling. Keeps them out of their mess and is gentle on their body.
Was she vaccinated for Marek's?
What are you feeding her? She appears to be a bantam. Bantams should be small. Feel her keel. What would it rate on this chart?