:hugs
If it is Marek's, culling the whole flock will not eradicate it from your property. Marek's can live a long time in the environment. Sometimes it may be necessary to put down a bird that is failing, but a lot of people learn to manage symptoms/give supportive care and keep a closed...
3.5' doesn't sound too high for me unless the landing area is very small (like they have to fly into the opposite wall).
Very understandable about the xray, vet care is expensive - that's why BYC exists and is so popular :)
Welcome To BYC, I'm sorry to hear about Louise :hugs
If you have photos those may be helpful.
You mention that her keel is going off to the side and she's in pain. Any bruising or sores like a breast blister on the keel? Could she have fallen from a high roost or been injured somehow...