Thanks everyone - you don't know how much these comments have meant to me. After speaking to the shelter again, and my vet, and taking a couple of days "time-out" (aka - time in cage) for the sake of everyone, Coco seemed different yesterday so... I let him out (yes I am a sucker for punishment). He climbed to the top of his cage and jumped over the step-up stick on to my back again. I froze (thinking oh, oh, here we go again) and he casually walked up my back to sit on my shoulder (which I don't allow) and started to preen himself! He nicely transferred to my hand and I gave him two almonds which he took happily. I kept him with me for about 10 minutes and then transferred him back to the cage (didn't want to try my luck). We had one more 10 minute outing with a reward last night and lots of "good boy" talk and clicking back and forth. I know you all told me he would come back but I really had to see it to believe it - a total Jekyll and Hyde. It lasted 10 weeks total (May through to July) - seemed to start and end almost overnight.
After much soul searching, he is staying with me. I had to seriously consider what to do for the best of all and yes, I admit that I considered giving him back to the shelter. It sounded sensible when I was at work in the office but when I got home and looked him in the eye, all sense went out the window. He belongs with me.
I am planning for next spring already. Money is a bit tight so I am trying to decide whether to get a new cage right now or a play/perch station. For some reason he is not taken with my tree. the idea is to get him on something lower so that he cannot jump onto people (as Jekyll or Hyde!)
His blood work came back and his calcium (which was a worry 6 months ago) is normal. the only issues now are low phosphorus (0.4 mmol/L and should be 0.9 - 1.8) and high bile acids - preprandial (119 and should be between 10 and 100)
The vet did not recommend milk thistle at this point as the other results were fine but wants him to switch entirely from seeds to pellets and to cut down on nuts and increase exercise (he is a perch potato outside of his human hunting season)
Hopefully the worst of it is over for this year and next year I will be better educated thus, better prepared.
Thanks again everyone. Your honest portrayals helped put everything into context.
Edit - I forgot to say his weight went up from 400 to 410 so he isn't starving but the vet said too many nuts are making him tubby. From now on I am supposed to hide the nuts and make him work for them, foraging. Problem is he doesn't seem to have made it past Piaget's object permanence stage - he watched me hide the nut but when it is out of sight, he loses all interest