You aren't backing up your points, only restating them.
As far as knowing the exact ratio between Carbon 14 and Carbon 12 (I'm using this as an example, most other radioisotopes have the same problem), we assume that the ratio of Carbon 14 to Carbon 12 in the atmosphere was a certain ratio thousands of years ago, but this cannot possibly be verified, because nobody can go back and directly measure that ratio.
Regarding outside influence, I would agree that the decay rates themselves cannot be changed by outside influences. However, outside influences can affect the ratio materials in various samples. If more C14 or C12 gets into a sample, then that would obviously skew the results quite a bit
