There are some obvious errors and omissions here.
Firstly if 1) is true we don't need 2) to be true. We can just count the number of remaining "parent" atoms. We don't even need to consider "daughter" atoms.
If 2) is true we don't need 1) to be true because we can calculate the original number of parent atoms from the number of "daughter" atoms and the remaining "parent" atoms.
What is more isochron methods account for the number or "daughter" atoms in the original state so we don't need 1) OR 2) to be true.
3) is true but can be checked for by inspecting the rock for signs of chemical or thermal alteration or other damage that might affect the results.
4) is true but there is as yet no relible evidence of a change in radioctive decay rates nor any plausible mechanism by which the decay rates would change significantly and still give consistent results over all the different methods of radiometric dating.
[Added in edit]
Here is the t.o. Isochron dating FAQ
Isochron DatingIsochron dating itself represents a check on 3) because the samples usually will not fall on the ascending straight line required for a true isochron if the samples have been affected in this way.
This message has been edited by PaulK, 09-03-2004 11:33 AM