I'm sorry for the delay on my end too - for some reason I could not 'lock' onto this question all day!
I wish I could say I was shocked about the treatment you found at that awful vet's office but I'm afraid it's not the first time I've heard something like this. What matters is that you did the right thing for your bird and I hope you never stop, no matter what kind of buffoons you come across in practice. Never stop trying
The only thing I'm seeing here that might need addressing is the amount of protein in your bird's diet. Let's try cutting back on that and increasing better sources of vitamin E and beta carotene along with calcium. To focus on this aim for fruits and vegetables deep orange and dark green in color. Sweet potatoes, squash, carrots; try broccoli, kale, spinach (this increases calcium as well)
Even most all natural yogurts are good calcium sources, sticking with the fruit flavors of course (no chocolate, coffee or stuff like that)
With all of this said, let me be perfectly honest, it may not help - whatever is going on might be something too far gone and only determined by a necropsy upon death; however, I'm in your court trying anything whatsoever and giving this bird every hope
So to sum up - for the next couple of days focus on vegetables and some fruits together with more calcium. If you see an improvement we'll know we're on the right track and can go from there.
I'm counting on you to check back with me ok?