It's so funny when vets try to given advice on nutrition because most of what they know, they learned outside of vet school, so you could have the same amount of knowledge as him... Ellie's vet said straight out that they learn very little in school and I could talk to a nutritionist and look online.
Nutro is the one blamed on all of those problems...
I have never seen any study that would indicate that high protein causes renal or liver issues but high protein shouldn't be fed if there is a preexisting condition. Whatever you're comfortable with... I personally lean toward lower protein but have no scientific anything to back that up.
Dehydration is very normal when they aren't allowed to have water before surgery. I wouldn't worry about it and if you want to be extra cautious, you could retest in a month.