She is showing classic Marek's symptoms. She is the right age for Marek's and she came from a private breeder. You have had other chicks of a juvenile age die..... all of these things point a long finger at Marek's being the cause. My gut feeling is that she has a tumour on her lungs causing the open mouth breathing.
Marek's compromises the immune system so infected birds can die from ailments that they would normally be able to shrug off or have developed resistance to. The paralysis Sassy is showing is more or less a text book symptom, but some birds with Marek's don't develop paralysis. Some die suddenly with tumours on internal organs or develop infections. Marek's birds are usually smaller and underweight and the disease also causes muscle wastage.
I had one that was nearly as bad as that and I nursed her through two attacks of it. The first she recovered after a couple of days, the second took months of supportive care and good food/vitamin supplements to support the immune system, probiotics to support the digestive system and sunshine and grass whenever possible. I would put her out on the lawn in a cage and scatter scratch in and around the cage so that the other hens came and foraged around her and at night I removed the cage and encouraged her to make her way back to the coop. She slowly regained significant use of her legs although never totally sound, but it was hard to watch her floundering during the first few weeks of this therapy.
You could make Sassy more comfortable by constructing her a simple chicken sling or hammock. If you do a search, you will find images of both quite professional but also simple DIY examples made with everyday items. You can clip/fasten little empty yoghurt pots onto the frame and put food and water in them so that she can more easily feed herself. Keeping her upright in a supported position will help both with her digestive system and breathing. I would give her the opportunity to fight it as long as she is showing an interest in food, but be prepared to end it when she will no longer eat as they go down hill quickly after that.
Good luck with her
Barbara
PS. There is every chance that she is feeling the cold even in summer. Sick birds often become hypothermic, especially if she is losing body condition, so providing her with a heat source will make her more comfortable.