I do apologize, but I'm a physicist and when I hear entropy, I automatically think "thermal processes can't be reversed without adding energy." Can you verify that you are indeed talking ONLY about a single definition of entropy? Is your definition of entropy this "information entropy" where an original message (or set of characters) is the perfect message and any deviation is a change in entropy?
If you're using entropy in the usual (non-information) way I can describe how DNA can exist and constantly be gaining thermal entropy through chemical and thermal processes and yet still exist unchanged because the food we eat replaces any energy lost. But if you're still talking about a strict computer-based definition of entropy, this would be quite off-topic.
if tests show a reduction of entropy within the letters(codons) of the nucliotide sequences...
I'm confused by the above quote. By the information standard of entropy, the only way to gain entropy (note that a gain in entropy is analygous to a "decay" of information -- a reduction of entropy is impossible in a truly closed system, but I THINK I understand that you meant the opposite) is to change or corrupt the message.
The problem with this regarding genetic diversity and sexual reproduction is that a perfect transmission of your genes is NOT the best standard for reproduction. In fact, cloning is horrible for a population (probably why even single-celled organisms often swap sections of DNA). We could talk all day about what exactly your definition says about a genome, but the key here is that the definition based on binary transmissioin of data does not apply directly to the sexual reproduction of a population.
In transmitting data, every bit must be transmitted perfectly for the information content to be highest. Computers run all sorts of checks to make sure that the data is correct and if it fails the tests, it is retransmitted.
In contrast, as I said before, when passing on genes the "best" method includes as much variation as possible while retaining the organism's function. No two creatures have exactly the same DNA, yet most survive quite nicely.
But anyway, I'm getting ahead of myself here. You're quite right, creationists are always looking for "entropy" gain, but the key to the discussion is that NO definition of entropy gain requires that a mutation be harmful to an organism. If it were a message from one computer to another, any mutation is harmful to the transmission, but a DNA sequence is not a message. The more a population's genome is varied the more able it is to adapt to an environment.
In essence, yes, if it were somehow important that DNA be transmitted perfectly, it would fall under the definitions based on computer transmissions, but since that is not the case, conclusions based on transmission of binary messages (like, "any signal degredation is bad") cannot be applied to DNA.