Taking into consideration all you have said, I find demodectic mange
(non contagious type) not likely as that usually strikes young dogs less than a year. Sarcoptic mange is contagious and a skin scraping should spot those critters pretty easily. Some vets will go ahead and treat for mange anyway, but I wouldn't in this case.
I also tend to not think it is a flea allergy since that would usually have been seen before this and while it only takes on flea to start a reaction, with the steps you have taken to eliminate them in the environment, I don't think that is likely either. You could use some Diatomaceous earth. Spread it in the carpets and let it work its way into the padding and carpet. It will kill fleas, larvae and other insects and is not a pesticide. You can use it on grass as well but it needs to be reapplied after it rains. You also need to wear a mask when applying as well.
Now it does sound like an allergy condition and the fact that it did improve at first with treatment is a good sign, but let me go over the problem with allergic conditions. You have an allergen that causes a dog to have an allergic reaction such as rash, inflammation, itchiness, etc. The dog scratches, damages the skin and the yeast and/or staph normally present on a dog's skin is not kept under control by the immune system and a staph or yeast dermatitis develops. The staph or yeast dermatitis in turn causes itching as well and keeps the skin red, irritated and eventually hair loss occurs as well.
Often this is treated with prednisone , antibiotics, anti-fungal shampoos and antihistimines. However, if the underlying allergen is not found and eliminated from the dog's environment, it continues to be a vicious cycle of reaction, scratching, skin infection and hair loss. So while allergy testing is expensive, given the age of your dog, you may want to consider it since it will identify the allergen. Now allergies can be to foods, pesticides, cleaning supplies even pollen and grass. Most vets will do a food trial but it can take 6 weeks or longer for a food trial to show improvement. If you switched her to a new food before this started or she had a different treat, etc, that might be the cause. In that case, your vet could do a food trial before testing, but given her condition, I'd opt for testing.
It might even be the pesticides in the grass or carpets. If you can identify things that changes before this occurred, you might be able to identify the cause yourself, but most of us can't pinpoint what changed. Let me give you some information on allergy testing. You can get a blood test or a skin test from a dermatologist.
Desensitization shots can be developed if it is an allergen that can not be avoided like grass.
I know I can't give you any magic cure for your dogs condition, but until the underlying cause is found, and then the skin dermatitis cleared, the cycle will continue.
I hope this information is helpful to you. If you need more information or clarification, please reply and I'll get back to you as soon as possible. If you are satisfied, please take the opportunity to rate.