While I doubt parasites are the main cause, I would still treat for them first. Its EXTREMELY common for only one rat in a group to show signs of parasites, while the others are infested but stay asymptomatic. Nonetheless all the rats have to be treated, and a drop of revolution per rat does the job great (you can get revolution from any vet). I've had cases of up to 12 rats in a group with parasites, and only one showed signs, but of course all needed treatment (which sucked but at least with revolution its an easy treatment).
The hair loss and ear scabs sound a lot like mites, but without a picture its hard to tell. I would still get them treated regardless though as I've seen mites get pretty bad in rats, and since you can't see them without a microscope its often best to treat for them whenever one rat has scabs/scratches in order to rule them out right away (and most of the time it ends up being parasites in my experience).
But again that doesn't explain the other symptoms - its does make since a bit, since you usually see mites causing symptoms in rats with already lowered immune systems. But it doesn't give you a reason for the other issues. Personally for those I would guess something like a pituitary tumor - I've had 3 rats with them, about 20% of all unspayed female rats get them and they are always ultimately fatal. Usually they come on quickly, but occasionally they grow slower and in fact my first experience with one was with my poor girl Latte - it took her months to develop symptoms, first off she had a stroke which caused partial loss of hind leg control (she also had parasites at the same time weirdly enough - at the time I actually thought her cagemates were hurting her because I couldn't imagine her weak hind legs causing scabs. But nope, she and the others had parasites and that was my first experience having rats treated for them. It was sadly the least of her incoming problems but thankfully it was easy to fix with vet help).
Latte ended up healing fully from the parasites, and even mostly from the stroke, but from there it all went downhill. A month later I noticed a slightly bulging eye, then some weird pushing behavior when her ears were scratched, then she started circling on occasion (around objects, not just in circles like a rat with an ear infection might), then she stopped being able to eat solids without help (because she couldn't hold them in her paws), then she lost the ability to walk, then slowly the ability to stand, and finally her teeth became misaligned and she could not longer even eat from a syringe, just barely being able to lick a bit of baby food stuck right in her mouth.
She also lost over half her weight in these 6 (nearly 7) months of progression - my poor baby was also put on cabergoline (a medicine that shrinks the most common types of pituitary gland tumors - sadly hers was the less common type that does not respond) and steroids (only briefly, but meant to tide her over by reducing inflammation and pressure on the rest of her brain), plus antibiotics due to the steroids effect on her immune system. Sadly it never was enough, and I really kept her alive too long. She probably would have lasted another month even, but she was basically comatose at that point, only reacted to light and food smells, hated her cagemates (would squeal at them for cuddling her), couldn't move, and at the end she also had several seizures. So I finally made the call and had her put to sleep - I wish I didn't wait so long, although I don't regret trying to help her. It was the most horrific thing I've seen in rats - I have seen all sorts of rat passings, but that long, prolonged one where Latte's mental capacities faded first, followed by her body, was by far the worst.
So please, please work with a vet and if needed, have your girl put down before she looses all quality of life. Honestly it sounds like its time now, and as I learned with Latte sometimes the best thing we can do do for our pets is to end their suffering before they loose all quality of life. You could see about steroids and cabergoline if your vet thinks its a PT, but honestly at this point I would strongly consider having her pts. I'm so sorry
Edited to add that I documented all my girls symptoms for my vet, and ended up compiling them to warn other rat owners of the symptoms. Latte didn't show every symptom, but she did show the most classic ones, especially the not being able to hold food in her paws (which is the most common symptoms of PTs) - its a bit disturbing to see, but nothing graphic. Maybe this can help you see if your girls symptoms match up - again I am so sorry this is happening to your girl