When it comes to eating grass I kind of side with SCM allowing the cat to eat grass. With that being said you don't want to do anything that's going to upset the stomach, or upset the stomach acidity especially with your cat suffering from renal disease. I'm enclosing a few websites that might be of interest to you to have look at. The first one is just an article on cat grass that is interesting reading. The following 2 deal with renal failure, foods and alternative medicines.
I hope these websites will shed some light on your problem, and hopefully some of the other members who have more experience in this than I have will add their comments too.
Regards the low protein, some of the vets are having 2nd thoughts on it. The vet that I am using for Jet's Renal Failure, handed me a can of prescription diet food and stated that she was not happy with the protein level in it, and suggested that I add some cooked chicken to it to increase the protein level.