This is why I find it so very, very difficult to make that final, loving decision for one of my brood. To me, unless the kitty is an old-timer, there's always that nagging question---"what if......?" I'm so glad Mackerel has shown some improvement and the vet feels more tests are in order. There are so many, many coming to you and your sweet girl; as others have said, maybe this will be one of those TCS miracles.
Keep fighting, Mackerel!
Keep fighting, Mackerel!