radioactive dating

When we know all of these:

• a rock contains A
• A decays into B
• A's half-life
• and both of these:
• how much A was in the rock when it formed
• how much A is in it now 
• or one from the above list, and both of these:
• how much B was in the rock when it formed
• how much B is in it now

we can accurately date the rock. But how do we know how much A or B was originally in the rock?

I should have gotten a handle on this a long time ago.

• some minerals strongly reject B, so we can be reasonably sure that all B found in such a mineral is from decay.
• in some materials, A and B would separate when forming (eg, from settling), so that all B found at A would be from decay. 

 Another explanation is still sketchy to me:

•  In the case of A being $C^{14}$, A is continuously generated by cosmic rays. So when we know all the carbon found in a rock, we know that a certain fraction of it was A ($C^{14}$).

My questions: Isn't that implying that the ratio of $C^{14}$ to all carbon has been constant over time? If so, how do we know that? And if the answer to that last question only tells us how we know the ratio of $C^{14}$ to all carbon has been constant in the atmosphere, how do we know that the atmosphere was the only source of $C^{14}$ trapped in rocks during their formation?