Sorry for your loss. A necropsy can be very affordable in some states, but you would need to call them and keep the body cold until it is done. Here is a list of state vets to call, especially if you have another chicken that has symptoms:
https://www.metzerfarms.com/poultry-labs.html
Could she have been injured by one of the other chickens? Is her head stuck down or limp? She does have neurological symptoms, which could be caused by Mareks, a head or back injury, or something else. Force feeding is not very safe. They can aspirate food and water and die. Tube feeding is easy...