I talked to the vet this morning in order to get a better sense of what is actually going on with Nikki's kidney values. It was a very instructive conversation, actually, and I'm glad she (this vet) is willing to actually go over technical details with me.
As for the nature of those details...apparently there IS actually some concern about her creatinine, even though it's technically within the normal range (2.1). The reason for this is the fact that her urinary specific gravity is lower than it should be.
The important lesson I'm taking from this is the fact that lab values can be kind of misleading if you just look at absolutes. Often you have to check and see how certain values relate to certain other values. Brodie's creatinine is actually higher than Nikki's (2.3) but since his urine was much more highly concentrated, this isn't indicative of a problem. The ability to concentrate urine is, it seems, a really significant function of feline biology and even an apparently small loss of this ability shouldn't be ignored.
...but all that said, the vet stopped short of actually officially diagnosing Nikki with chronic renal failure (CRF), though it certainly sounds to me like that's what we're looking at long-term. Which doesn't freak me out nearly as much as one would think it might, probably because I've known some cats-of-friends with the condition who've nonetheless gone on to live to ripe old ages. It's not a death sentence, nor is it the sort of thing that means the cat is going to be living for years in constant pain.
Moreover, there's no way to know how fast it will progress...the vet said she used to see a 23-year-old cat who had numbers very similar to Nikki's (and who'd had those same results for something like six years in a row). Of course I'm going to watch Nikki more carefully now for signs of discomfort and pay more attention to things like making sure she stays well-hydrated, but I don't see any reason to treat her with pity or what-have-you. She'd hate that, and it just doesn't seem logical besides.
As far as treatment goes...the vet did end up recommending I try offering Nikki some prescription food. There are different "levels" of RX for kidney trouble, apparently, and not all of them are extremely low in protein. She's going to leave me a can of Purina kidney diet this afternoon when I go and pick up the Panacur (giardia medication) for the younger kitties. Nikki is the pickiest eater I've ever met, so even if she's not at the stage yet where she absolutely needs a prescription diet I figure it's worth seeing how she reacts to it. And I was relieved to learn that there are other options if she refuses the RX food, e.g., mixing in a phosphorous-binding powder with her regular food.
She's not yet at the medication-needing stage, mind you, but when and if she gets to that point they will probably try an ACE inhibitor first. Either way, I'm just...really glad that veterinary care even exists, and that nobody is trying to pressure me to just "put her down". Nikki is a tough kitty and while she's never been much of a happy-go-lucky sort (except in case of copious sunbeams) I am absolutely certain that she has a whole slew of very important reasons for living all her own.