Hi MC,
While I was at my vitamin store today, the owner told me about a customer of hers who has a cat with crystal issues. The woman does feed wet only, though the store owner wasn't sure if it was grain-free, and she's been giving her cat 1/8 tsp (600mg) of non-buffered ascorbic acid in the cat's food 2 times a week. The cat's crystal issues stopped. I asked what kind of crystals the cat had (e.g. calcium or magnesium) and the store owner didn't know.
Interestingly, the book I have on natural remedies for cats also mentions vitamin C for urinary issues, though it doesn't specify which crystals to use it for, though perhaps it's good for both. I have to be honest and say the book is from 1999, though. That said, I know vitamin C is anti-inflammatory, and perhaps it helps to acidify the urine (I'm guessing it's called "ascorbic acid" for a reason
).
Anyway, I wanted to toss this info out there to you and anyone else so you could do some digging if you wanted to.
P.S. I know you weren't confident about the testing strips that you were using to test Mazy's urine when she gets the ACV, but I think you can find better strips (that go down to a lower number) on Amazon, if you decide you want to try testing the pH again.