First of all, I am also not a veterinarian and never had any formal schooling on anything remotely related and what I know it's from experience (both personal and from other people who ask my opinion on their birds problems and provide me with X-Rays, blood work, prognosis, treatments, etc) so don't take anything I say like the gospel but I have some questions and comments:
Has the doctor considered liver failure? I thought she did but will ask. Has he had a bile acids test? (no) Because it's the only way to know how the liver is working and, in my personal experience, all older zons have liver problems. High chloride and low phosphorus are both indicative of it PLUS the new X-ray seems to show (in my personal and uneducated experience) an enlarged liver as well as a bit of fluids in his abdomen. hhmm... dr. said that organs looked normal.. another q for the dr.
Has he ever had heavy metal poisoning? Because the old X-Ray appears to show metal in the gizzard and that causes A LOT of damage to the liver I asked the vet about that (other vet did old xray) and he didn't think it was metal... Toots has never had metal poisoning. I will show the xray to my present Dr. and see what she says - damage which needs to be treated with supplements for the rest of the parrot's life -which you never did mention when you explained his diet and, by the way, his diet seems to be very high in protein and very low in fiber (the fiber they add to pellets has been found in tests not to work for birds although it's very good for mammals) AND also very little phytonutrients and enzymes (you can't get them in pellets, they need to come from fresh plant material). This is not good for any parrot but, most especially for zons which require a diet of mostly fresh plant material and low protein.
Parrots with acute liver damage would also show high monocytes as these are used for inflammation and cannot take lots of medicines (antibiotics, pain killers, anti-virals all go through the liver, antifungals been especially bad for it) so his deterioration despite all the medicines would make sense if this is what is wrong with him.
Also, liver failure kills because the body is filled up with toxins the liver can no longer get rid of... it's like slow poisoning - and they show the symptoms you talk about because their motor skills become terribly impaired due to the toxins. They become incoordinated and like whoozy-headed, often tripping, not been able to hold things with their feet, etc.
Again, I stress: I am not a vet but I have several birds with advanced liver damage, some due to bad diet, some due to disease (like chlamydiosis) and some post metal poisoning so I've gotten a bit of experience with the problem and, although a bile acids test is not cheap (prices vary but it goes for around $125-140), it is the ONLY way to accurately measure liver function.
I'll be watching...
