Hello and welcome to the forum. It's a dilemma when your own assessment of the situation differs from your vet's. However, as long as you both have an understanding then you should be able to work together on this problem. From my own experience of renal failure in humans, the majority require a low protein diet simply because the kidneys excrete the waste products of it so these tend to build up in the blood stream resulting in high urea and creatinine. There are some renal diseases that require higher protein.
Normally canine renal failure it is age related (as with your dog) and the thinking is to relieve the burden on the kidneys by following a low protein diet. However, your dog is not getting on very well with the prescription diet and you have said in the past he has had problems with different foods. I therefore won't go down the road of suggesting anything else.
As you say, your dog now has a limited life span. When my dogs have been in this situation at some point I have taken the view that it is the quality of life that matters, not the quantity. It is good that your dog likes the raw diet and that you have now sourced a local stockist of Natural Instinct. If you feel that your dog's quality of life will be improved on raw diet then it is your choice and decision to make. Explain to your vet that you are looking at this as good palliative care, which it is in a way because his kidneys will continue to deteriorate over time. If his bloods do go downhill then at least he has had the pleasure of enjoying his food (very important for dogs). You could perhaps consider telephoning NI to ask for advice.
The only other thing that occurs to me is to try one of the air or freeze dried raw foods such as ZiwiPeak
. It is high protein. I don't have any experience of these products but there are a few listed in the Dog Food Directory of this website.