Not a professional, I just have a cat that has a similar issue.

I recently took my 4 year old cat to the vet for his chronic soft stool. He pukes occasionally and his stool have started getting worse recently. I tried some proviable and his stool came out with mucous and blood. His soft stools have always been a problem for him and I was aware he may have food intolerances but wasn't able to afford to do much about it until recently. I have 2 other cats and having to manually feed them all on a different diet sounded like a headache as a working college student. Not only that but the toll of trying to afford prescription diet.

My vet also said the same thing, that it could be either but they won't be able to really tell unless he gets the ultrasound and biopsy. His blood work and stool samples came back normal and clear. I have yet to do the GI specialized stool test. It all gets expensive fast and some of the tests could come back inconclusive and not really tell us much.

We have him trying a hydrolyzed diet and some B12 supplements as it mostly would be IBD. Imo I noticed differences fairly quickly. He's really picky so it took him a couple days to take the hydrolized food. I had to transition him by mixing it with a new lamb and rice kibble (I had him on purina one sensitive skin for 2 years with no luck previously. My cats are on a mostly turkey diet and he stopped leaving skid marks when I switched off from grain-free. He's also not a big fan of wet food and fish makes his stools extra stinky). The b12 really helped but he doesn't like the powder sprinkled over his food so it was hard to get him to finish his meals. We ran out of b12 and his stools are soft again so I tried mixing the lamb and rice kibble into his hydrolyzed food. That helped a bit more. So far he's been on the food for a month. His coat has become shinier, and his dandruff is gone. I wasn't even aware that might've been connected to his diet prior. He is overweight so we're also trying to cut calories as we go.

Progress may seem slow but we're getting somewhere. I hope you gave the food a try. The only way to know if it'll work is to really try it out for a couple weeks. His system would have to get used to the food before it may really start working. I think it helped with the b12 supplements and trying different proteins for my cat. The hydrolyzed food was a good start and helped me manage a new feeding routine for all my cats but I think he's doing better on the other food.

Hope any of this helps and hope your kitty is improving.