(1) Carbon dating isn't used to date rocks.First of all, let me give you one that is obvious and you probably already heard it. Carbon dating. About how long it takes for something to decay. I'm not even close to a physicist. Now how do you know how much carbon was in that rock when God created it? God could have created any amount of carbon in that rock he wanted.
(2) If God didn't want us to think that the Earth was old, He could have either made the isotope ratios in rock either be completely random (which would cause all isotopic dating methods to be inconsistent with each other) or He could have made the isotope ratios line up with that of a young Earth.
(3) Isochron plots do let us know what the original isotope ratios were.
(4) When different dating methods consistently yield the same age range for a given rock layer, you know it can't just be a coincidence. Either the age is correct or God carefully crafted the isotope ratios so that it would appear to be a certain age, thus making Him deceptive. I prefer to think that He would have no need to do this.