Young cat with chronic diarrhea, weight loss, inflammatory bloodwork and high ALT — worried about FIP while waiting for GI specialist

Species: Cat
Age: 8 months
Sex/Neuter status: Female, intact
Breed: Neva Masquerade
Body weight: Was around 2.5 kg, increased to 2.8 kg after treatment started, now back down to around 2.6 kg
General location: Europe

History:
My cat has had chronic loose stool/diarrhea for around 2 months. She is currently under veterinary care and has been referred to a GI/internal medicine specialist, but the appointment is not for several weeks. I am not looking for a diagnosis online, but I would really appreciate help understanding what questions/tests I should discuss with my vet while we wait.

She has had periods of reduced appetite and weight fluctuation. She does not vomit. She is still playful at times and can seem fairly bright, but she has episodes where she seems uncomfortable, lower in energy, and possibly painful in her abdomen.

She is currently on day 14 of prednisolone and has also been given Mirataz as an appetite stimulant. She is eating only a hydrolyzed hypoallergenic diet right now.

During the first week on prednisolone/Mirataz, she improved a lot. She ate better, gained weight from about 2.5 kg to 2.8 kg, seemed happier, and her stool started to look slightly more formed. Over the last 4–5 days, she has worsened again. She is now back down to about 2.6 kg and still loosing, has reduced appetite, and goes to the litter box several times per day.

We also worry she may have intermittent fever because she sometimes feels warmer than normal, but rectal temperature has not been possible because she becomes extremely distressed when anyone tries.

Clinical signs:

• Chronic loose stool/diarrhea, around 2 months
• Sometimes mucus in stool
• No obvious blood seen
• No vomiting
• Reduced appetite again despite initial improvement
• Weight loss after initial weight gain
• Several bowel movements/litter box visits per day
• Suspected abdominal discomfort/pain
• Possible intermittent fever, not confirmed
• Still playful/bright at times

Tests performed so far:

Fecal/parasite testing:

• Currently negative for parasites, including Giardia, Tritrichomonas, Cystoisospora/coccidia, Cryptosporidium, Toxoplasma, Toxocara and other worms/parasites included in the tested

Respiratory/eye/oral infectious PCR testing:

• FHV-1 / feline herpesvirus PCR: negative
• Feline chlamydiosis / Chlamydia felis PCR: negative
• FCV / feline calicivirus RT-PCR: negative

CBC:

• WBC: 24.58, reference 2.87–17.02 — high
• Neutrophils: 17.50, reference 2.30–10.29 — high
• Monocytes: 1.14, reference 0.05–0.67 — high
• RBC: 9.25, reference 6.54–12.20 — normal
• Hematocrit: 0.348, reference 0.303–0.523 — normal
• Hemoglobin: 125, reference 98–162 — normal
• Platelets: 546, reference 151–600 — normal/high-normal
• Plateletcrit: 0.89, reference 0.17–0.86 — slightly high

Chemistry:

• ALT: 686, reference 12–130 — very high
• Globulin: 53, reference 28–51 — slightly high
• Albumin: 28, reference 22–40 — normal
• A/G ratio: 0.5
• SDMA: 15, reference 0–14 — slightly high
• Creatinine: 76, reference 71–212 — normal
• Urea: 7.0, reference 5.7–12.9 — normal
• Phosphorus: 2.27, reference 1.00–2.42 — high-normal
• Bilirubin: 5, reference 0–15 — normal
• ALP: 66, reference 14–111 — normal
• GGT: 0, reference 0–4 — normal
• Amylase and lipase were within reference range

Abdominal ultrasound:
The ultrasound report described mild changes in the duodenum, possibly suggesting enteritis. The descending/transverse colon had mild thickening/bumpiness, consistent with colitis. The gallbladder wall was mildly thickened/hazy, with differentials including cholangitis or gallbladder wall edema. Some intestinal lymph nodes were mildly enlarged and thought to possibly be reactive. The liver, pancreas, kidneys, spleen, adrenal glands, urinary bladder and the rest of the GI tract were reported as normal.

My main concern:
I am very worried about FIP because of the chronic GI signs, weight loss, inflammatory bloodwork, globulin being slightly high, A/G ratio of 0.5, and the fact that she improved at first on prednisolone but is now declining again. My vet has said FIP is not their first suspicion because she is still fairly bright, but I am struggling to understand how likely or unlikely FIP is with this pattern.

Questions:

1. Based on these findings, how strongly should FIP remain on the differential list?
2. Does an A/G ratio of 0.5 with slightly high globulin make FIP more concerning, or is this still nonspecific inflammation?
3. Can FIP present mainly with chronic diarrhea/colitis-like signs and a cat that is still bright/playful at times?
4. How concerning is an ALT of 686 when ALP, GGT and bilirubin are normal?
5. Could this pattern fit IBD/food-responsive enteropathy, cholangitis/hepatitis, infectious disease, or another inflammatory GI disease instead?
6. While waiting for the GI specialist, what additional diagnostics would be reasonable to ask my vet about — for example repeat CBC/chemistry, urinalysis, B12/folate, bile acids, Spec fPL, fecal culture/PCR, FCoV antibody/PCR, or other FIP-related testing?
7. At what point should I consider this urgent/emergency rather than waiting for the specialist appointment?

I understand no one can diagnose her online and I am not asking for treatment instructions. I mainly want to know what I should ask my vet about next, and how worried I should be about FIP versus other causes while we wait for the specialist.