OK, where to start. First of all, you can't diagnose lymphoma vs IBD with a fine needle aspirate. You have to do either a biopsy or endoscopy. I am sorry they didn't do a biopsy when they did the feces elimination since they would have been right there.

GI inflammation can be caused by either IBD or SCL (small cell lymphoma). Due to Neko's heart condition, she could not undergo anesthesia, so could not get a proper diagnosis. Has Dimmu been losing any weight lately? Has the vet suggested a GI blood panel to find out his cobalimin/folate levels?
I read about lymphoma diagnose after they have done the ultrasound, and really regret I didn't know about it earlier to press for a biopsy.

Dimmu's weight is stable, but the nutritionist we saw yesterday said he should lose some weight. We were surprised, Dimmu has no fat on his back, around his ribs and on his hip, he's actually very lean/almost skinny in these areas, but he does have a fatty belly. We were told by all the vets we have seen that he should put on some more weight, the nutritionist is the only person that thinks he should lose weight, and she wanted to put him on Royal Canin Obesity Management wet food (
https://www.royalcanin.co.uk/products/cat/feline-veterinary-diet/obesity-management-wet/) which has 19.4g carb in the dry matter. It seems like a lot to me, and I am not convinced of the ingredient list, either. She also said we should only feed him twice a day and totally nothing in between, I smiled and nodded. I agreed that he has to lose the belly fat, but if he is to lose 10lbs as she wanted him to, he'd be super skinny. I want him to lose the fat around his belly, should I start by cutting down a little of his food intake?
The vet wanted to see him back in about 3 to 4 weeks to check his GI health and do another blood work to see if his RBC comes up. Should I ask for a GI blood panel, too? He gets 4mg of B-12/day and is also on B complex, anything else I can do to help his anemia?
Next, the raw food does not contribute to IBD/GI issues. If there are allergies to proteins (IBD), it's the specific protein you need to address. Fortunately with raw food, there are lots of protein options out there. One thing you might want to consider is a raw diet that used eggshell calcium instead of bones. Some cats have problems passing bones. Here's a good web site on
raw feeding for IBD cats.
I didn't think it was the raw food that cause the IBD/GI issues, either. But he's on a rotation of many different sources of proteins that it's difficult to tell which ones he might have difficulty with. Maybe I should start with what used to work for him again and slowly add/eliminate other options one at a time. He clearly has problem passing bones because the vet found lots of it in his feces. So will look into the eggshell calcium, too. Another option I am wondering is switching him to wet food for a while until his constipation problem is resolved, and then re-introduce raw back, thought?