Your breeder has a registered cattery name with TICA. Basically, that's all it means. Some catteries get inspected and get to put on their websites they have been inspected. Anyone can pay a membership fee to TICA. However, as a breeder member, you must agree to obey their guidelines (which are on their website somewhere, I'm sure).
When it came to Raiden -- he started losing weight rapidly and was vomiting up to five times a day. Took him to his regular vet and they referred us to an internal medicine vet. We did the ultrasound and ran blood, urine and fecal tests -- only a higher white blood cell count was present. They recommended scoping him from both ends and doing biopsies. With his age and heart murmur, I didn't want to start with anything invasive, so we did the ultrasound which didn't show any lesions. The internist was keen on talking about lymphoma as the issue. We decided to start him on medication and see how he did. He gained a lot of weight (6 pounds). Most cats with cancer are losing weight. We got the vomiting stopped and the poops solid again so we cut back on the prednisone dosage and the cut back the metronidazole to once a day. Two months later, he was back to vomiting frequently. Back to our regular vet who raised the dosage of prednisone back to where we started. Well, the stools became liquid again, so I went back to the twice a day routine with the metronidazole. That's where we are now. They are checking his B12 levels (for the third time) (B12 helps with absorption of the nutrients) and waiting on those results. Raiden poops once a day, so this is not like constant diarrhea. He eats like a pig, behaves like a normal bengal.
I'm not really sure what a specialized fecal test may entail. Most vets can check for regular parasites, but the TF test has to be done outside the vet's lab. You've already had that done. Maybe a specialized test to see what the "content" is made up of. I don't know. It just all adds up and if you cannot get this solved rather quickly, this is something you are going to have to deal with and pay for -- for a very long time.
I'm sorry your breeder has abandoned you. Most do not and many will take the cat back and give you another kitten. In most cases, the owners have fallen deeply in love with the kitten and do not want to give the kitten up. Trixie should have spent at least another four weeks with the breeder and mother. What you are going through is not common at all. It does happen, but the vast majority of breeders sell healthy, socialized kittens. There must be a reason that you were gifted Trixie. Bless you, bless you.