Did they do the fecals inhouse or were they sent to a lab? There should be two separate samples. One is for parasites, the other for bacteria etc. The ones done in house only catch the obvious so you can't rule out parasites or an intestinal infection if the only testing was done in house. If samples have not been sent out, that would be my first step. To do the samples, she needs to be off all antibiotics for 3-4 days.
By diarrhea do you mean full on liquid, or very soft, mushy? If the latter, it may be because her digestive system is upset with all the different foods.
Have you tried different kinds of canned cat food? I have a little guy who came to me as a rescue/rehome. I won't go into all the details except to say, I had to syringe feed him or he simply would not eat. The only thing he would eat on his own was scrambled eggs but wouldn't touch them two times in a row. For 3 months I syringe fed him while trying different types of canned and kibble which he would not touch. Being on a soft diet, his poop was almost always a mess. Sometimes it would firm up, but then back to the mess and he was vomiting on occasion. Then I bought Hills Natural Balance Chicken and rice canned food to try. http://www.hillspet.ca/en-ca/ideal-b...-cat-food.html
He ate it, and continued eating it licking the dish clean each night. After two weeks on the canned I decided to try the kibble and OMG, he ate it too and now will only eat the kibble. This is a guy who I thought I'd be syringe feeding for the rest of his life. Parker was legitimately sick when he came to me which is why he came to me, but even after he was better, he still would not eat on his own. Now that he is eating on his own, he no longer vomits. I am unsure if before the Hills was there really no other food that he liked, or is it just a coincidence that it took this long for his intestinal system to fully heal from the infection he had.
I'd keep trying different foods and see if you find something she likes. Try putting the food in bed with her.
Poor girly and poor you. It's difficult when we don't know what is wrong and how to help them.