Really something to ask the vet. I understand you wanting to be sure the vet did all they could in the situation they had but try not upset yourself.
Page 608 of link listed in previous post lists African Gray 3rd column so it does vary with species.
HB 14.2 to 17 if HB is HGB I don't see HGB
Range of parameters measured on the Avian CBC with the range of normal values | Beauty of Birds
Don;t see HGB or HB on Pennys Hematology report
Calcium 9.4 Says low CA can cause seizures normal range 8.2 to 10.4
Penny had low cholesterol, low AST, low bile acids but vet did not consider it a potential problem. Have to watch that as it could signal a liver issue.
Was a necropsy ever discussed. Even if one was done sometimes it does not come up with a definite cause. There may not have been one cause but continual absorption of zinc over the years could have contributed. I imagine just zinc dust in the mouth & on the tongue not swallowed paint pieces could over time build up toxin.
We can only speculate at this point. Are the cabinets old? Could there be lead paint under new coats? Its good to keep your mind occupied & would be good if you knew so in the future should you get another bird you could avoid the danger. We can't control everything. To give them a good life means giving some liberties or else we'd cage them all the time & even then there are dangers in food & the cage itself. We just try to do the best we can like you did.
Sorry for some reason I though it just happened. Still its good you do go over everything in the hope of finding some closure. I think you will know when you are ready for another bird.
I would have done the same thing under those circumstances. You did the best you could for her in life & the best you could do so she would not suffer needlessly before passing.