Have you tried starting her on a good quality canned food yet? I'm not talking the stuff vets sell that passes for food (barely). I am talking Kitten Wellness or another grain free, byproduct free kitten food. You could start her off with a small amount watered down so she can digest it better. If she doesn't know what to do, and probably won't, try sticking your fingertip in the food and putting a small amount on her nose. She will soon associate that taste with what's in the dish.
I have a feeling with all the changes she's been through plus being taken off mom's milk so early the diarrhea should be considered normal. If she has been to the vet a couple of times already and has been dewormed and has had coccidia meds then I would expect a little diarrhea.
Keep an eye on her and make sure she stays hydrated. You are doing a wonderful job so far.
And thank you for bringing this little one home. Was she an only one?