There are seven kittens in total

Hello Doctor, I am seeking an urgent second opinion regarding a group of seven approximately 6-month-old kittens that have become seriously ill. There are seven kittens in total. The illness started approximately 5 days ago, and unfortunately, two of the kittens have already died. According to the veterinarian who previously examined them, a feline panleukopenia (FPV) test was positive. Clinical signs observed The affected kittens have developed several of the following signs: Severe lethargy and weakness Fever Complete or severe loss of appetite Vomiting Diarrhea The diarrhea was initially yellow and later became green and then dark/blackish Markedly reduced urination Increased water consumption Oral pain and significant lesions/inflammation inside the mouth Difficulty breathing in some of the kittens Progressive yellow discoloration of the eyes, which makes me concerned about jaundice Severe deterioration over several days Two kittens have already died I have also noticed severe inflammation and abnormal tissue in the mouth. In one kitten, there appears to be an unusual piece/layer of tissue covering much of the front teeth. When the upper lip is pulled back, this tissue appears somewhat detached or separate from the underlying gum rather than being clearly continuous with it. I have not attempted to remove it. I previously took several photographs of the oral lesions and can provide them if helpful. Current concerns Because of the positive FPV test and the gastrointestinal signs, feline panleukopenia seems to be an important confirmed diagnosis. However, I am concerned that there may be another concurrent disease, because of the following combination: severe oral lesions, respiratory difficulty, progressive jaundice/yellow eyes, and the severity of the disease affecting multiple kittens. I am particularly wondering whether feline calicivirus (FCV), severe systemic calicivirus infection, FIP, secondary bacterial sepsis, hepatic disease, or another concurrent infection could also be involved. I understand that these cannot be diagnosed reliably from symptoms alone. Important additional information At the moment, unfortunately, I do not have access to the previous blood test results, CBC/biochemistry results, or radiographs. I am trying to obtain them. I would especially like to know what diagnostic tests you would recommend urgently, considering the current situation. If possible, I would appreciate your opinion regarding: Whether the clinical picture is consistent with severe feline panleukopenia. Whether the yellow discoloration could represent jaundice, and what the most important differential diagnoses would be. Whether concurrent feline calicivirus infection could explain the oral lesions and respiratory signs. Whether FIP should be investigated at this stage and, if so, which tests would be most useful. Whether the oral tissue described above could represent severe inflammatory/proliferative tissue, necrotic tissue, sloughed mucosa, or something else. Which blood tests and imaging studies should be prioritized. What emergency supportive treatment would be appropriate while the diagnostic work-up is being performed. The kittens are very young and several are critically ill, so I am particularly concerned about dehydration, electrolyte abnormalities, hypoglycemia, anemia, secondary bacterial infection/sepsis, and liver involvement. I would be very grateful for any guidance on what should be done immediately and which diagnostic findings would help distinguish severe panleukopenia from concurrent FCV, FIP, or other systemic disease. I can also send photographs of the mouth, eyes, and any other visible clinical signs. Thank you very much for your help.