Now I have to say that I am concerned to hear that Ally has had such a chronic case of diarrhea. And with her stool samples ruling out infectious issues and her bloods ruling out systemic/organ troubles; I would be most concerned that we have more then just a dietary sensitivity especially if the special diet has not totally settled this for her. Specifically, my two major concerns for Ally's signs would be an inflammatory bowel disease type issue within her intestines or a possible GI tumor (lymphoma).
With those 2 concerns in mind, I would note that diagnosis of either usually does require further diagnostics to catch them. As I noted with my questions, we often get an idea if one of these suspects is present using ultrasound (as they often cause gut thickening). From there, we can sometimes diagnose which one we have using ultrasound guided gut biopsies; but usually will use a scope with a camera to look at the details of the gut from the inside as well as take biopsies. So, those would be the next diagnostic steps to consider for Ally.
Otherwise, in regards ***** ***** weight loss, we do have a few considerations. Just having diarrhea (since there is a reduced absorption of her food/nutrients with this ) can cause secondary weight loss. And progression of our differentials could also lead to that malabsorption worsening and thus causing weight loss to become more prominent now. That all said, I suspect it is a combination of these plus her current diet causing weight loss here. While she is processing her bland diet better it will have less calories/fat per bite then commercial foods would have. So, that is likely why we are seeing a drop in weight even with an improvement in her stool.
Therefore, in regards ***** ***** counter this, we'd need to think about either feeding more (so she gets more calories/fat per day) or supplementing that light diet with a a nutrient support. There are nutrient pastes (ie Nutrical) that could help or you could try her with a easy to digest liquid diet (ie Clinicare, Dogsure). Or we could try looking for a limited ingredient commercial diet with a fish/potato base (ie James WellBeloved, Eukanuba, etc).
Finally, while potentially not necessary if her stools are looking good, I do want to note some stool firming aids. First, you could bulk up her diet with fiber by adding canned pumpkin, all bran or even a 1/2tsp of unflavored Metamucil to her meals. Or there are some good OTC probiotic/kaolin pastes (ie Fast Balance, Protexin Prokolin, Canikur, Propectalin) that you could try with her. They would help make sure her gut microflora population was in good stead and the kaolin can help firm up feces These can be purchased from your vet, pet stores, and even online at sites like Amazon. So, those could be considered as a maintenance treatment for her.
Overall, these would be my concerns with Ally's persistent loose stools and weight loss. So, we should try to get some more calories into her and consider these other treatments +/- testing to help keep her weight stable and help get to the root of her signs.
All the best,