There's 4 different sub-species of YNA. Yours looks to be a parvipes. goggle YNA sub-species. All depends on where in their native range they came from. parvipes are a bit smaller than most with noticeably smaller feet. Also about the only nape that gets the yellow collar around their neck when FULLY mature. I'd guess your bird to be 30 yrs at least.( does he have a open type USDA quarantine band??) Also about the right age to suffer from VitA deficiency from a seed diet. (Surprised your AV didn't recommend something for that) VitA issues usually show up in the liver. It suppresses their immune system and leads to infection in mucous membranes and surrounding organs. The liver is often enlarged and is known as "fatty liver" disease. This could very well be the cause to "barbering" or plucking under the wings. ( they're trying to get at what'a bothering them on the inside). The legs are a different matter , we've had several threads about metal toxicity lately,, causing zons to pluck,bite and even mutilate their legs. Both of these conditions can be treated.
The right AV would deal with these issues often in older zons. If you've not sure about your AV, PM me. I do know some other AVs South of you who are the real thing and charge alot less than 300$. What Vet does your zoo use? I'm sure he could take blood/fecal sample and other swabs for culture. There 's a good AV (only) who could do the cultures and test for a fraction of that , if you could send them to him. He deals with these issues on a daily basis and has his own lab/pathologist. (he does zoos, inspections for the USDA, breeder operations,etc). If nothing else your zoo vet could consult with him.
A AV who recommends a collar for a zon is an idiot IMO. Very rare for a zon to pluck for any reason other than a medical one. of course a good AV would know that. Fix the medical issues. The plucking will go away.
The foot issue, goggle "bumble foot" in parrots. perches are most likely to be your issue. Get rid of any "dowel" type perches,(small diameter,smooth, dirty). I'd go to "bird safe' natural perches. Some of the best in the world grow wild in your area. The sore spots on the feet are caused by wear/pressure on the same area of the foot. natural perches vary in size and shape so that different parts of the foot are used when the bird moves around on the perch. If he spends alot of time on the rope perches,,,they are just as bad, moreso because the birds like them. Perch diameter should be 1 1/2 times the dia of their foot. Example, if his toes touch on a 1" dia perch ,,,then he needs a 1 1/2" dia perch. Toes should only reach less than 75% of the way around a perch. Heck, a 2 or 3" diameter natural perch would be good.Zons need different sizes, generally larger is better.
I could write a book here but will try not too. Loads of good info in the older threads, use the forum search near the top right to look up some of these issue. PM me if you need help or info.