Age dating question

Thanks for everyone's input on the other thread regarding mutation
traits in sexual reproduction; those are quite a bit of information and
it'll take some time for me to digest them. For reasons explained
previously, I have another question regarding the age dating for
objects or fossils. Now, I understand that the fact is there are
several methods for dating regarding, say, the age of the earth, or a
particular fossils, and these methods typically all agree with each
other on the results. These methods commonly involve the calculation of
measuring the amount/ratio of element A and element B, where element A
would naturally decay into B, thus by calculating the ratio and the
rate of decay, one calculate the age of the object in question.
(Ignoring the on a cosmo scale which uses other methods)

A rather popular arguement that keeps on repeating itself on various
boards is that such methods require a constant rate of decay with
knowledge of starting condition and no outside influence, etc. I saw an
analogy somewhere as follows: "A person (#1)walks into a room and
observes another person (#2) reading a book. The book opens to page 100
at the time of entrance of #1. One minute later, #2 turns the page to
101; another minute later, #2 turns the page to 102. #1 then conclude
that prior to #1 entering the room, #2 has been in the room for 99-100
minutes. Since (1) the person might not start at page 1, (2) the book
might have pages with fewer/more content and thus require fewer/more
time to read, and (3) person #2 could be distracted by #1 somehow
during the observation, or there are some circumstances to help/hinder
#2 during the reading time previously, the method cannot provide a
reasonable analysis of #2's time in the room reading."

I am trying to find a suitable counter-arguement for this.
Specifically, are there any way of determining the following to ensure
the decay-type of dating method?
1, the original composition of elements as starting condition;
2, the rate does not change over time;
3, the fact of observation and measurement, and the relevant conditions
during an object's life time, would not influence the results.

Thanks in advance.

.

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