chronicrain,
11 years is quite old for a sugar glider and your recovery methods are spot on. If you have not introduced any new animals to her recently, I doubt the spot on her head is parasitic in nature. It is much more likely that she has developed an allergy that is causing hairloss, especially as you are noticing itching. You can safely use topical benadryl creme on the area to see if she stops itching and the hair starts to regrow.
As to the 3rd eyelid, I suspect you are seeing it because she is still a bit dehydrated. As she is drinking well, I would continue to supplement with pedialyte and increase her fruit consumption a tiny bit to help her fluid intake. If she doesn't respond to that, she may need fluids given subcutaneously by a vet just to give her a boost. At her age, I'm concerned that she might have some kidney issues, though it is hard to tell and getting bloodwork on these little guys is very stressful to them.
You are an excellent caretaker, continue the good work,
Dr. Karen
You are quite correct. I agree with you that the bald spot is much more likely to be stress or nutrition than anything parasitic or contagious.
Veterinarian
1992 graduate of Cornell University Veterinary School, small mammal interest