Did you read the text on Wikipedia I linked to?
It says there that the speed is relative to the total amount of isotopes in the sample because the probability of an 1 isotope decaying in a very small timeframe is bigger when there are more isotopes.
Because the moment at which an isotope decays is random. Follow the formulas and you will see that they are correct.
The C14 dating method is usually used to date dead things that once lived. Idon't really understand why anyone would use it on a rock but there might have been reasons for that?
Living things take in C14 from the air while they live but not when they are dead. If you read the Wikipedia article on radiocarbon dating then you will see that a constant value of C14 is only assumed for approximate analyses and that callibration curves are used for more precise analyses. These curves precision can indeed be questioned.
And I'll say it once more, radiocarbon dating is only one of many forms of radiodating....