I agree that Biblical interpretation can be argued endlessly, but I would point out that critical thinking is an established discipline.
I don't think that what does and does not constitute a valid logical argument is open to debate.
An argument is valid if and only if the conclusion follows inarguably from the premise(s) In other words, as long as the premises are true, the conclusion must necessarily be true as well.
Here is a valid argument:
Tom is over 50 years old. Therefore Tom is over 20 years old.
Since it's mathematically impossible to be over 50 and not be over 20 as well, the premise guarantees the truthfulness of the conclusion. If the premise is true, the conclusion is just as true.
Here is an invalid argument:
Tom is over 20 years old. Therefore Tom is over 50 years old.
Since it is certainly possible to be over 20 and still be under 50, the premise does not guarantee the truthfulness of the conclusion.
This is not something that is open to debate. No amount of arguing will change the fact that the second example is a flawed argument.
The more premises there are, the more complicated this can become, but the litmus test of an argument's validity does not change.
An easy and fun test is available here: http://www.think-logically.co.uk/lt.htm
At first blush, this might not seem particularly relevant to the JW teaching on blood, but here is the connection:
The JW teaching on blood originated with a very simple argument:
A. The Bible forbids the eating of blood
B. Transfusion is a modern way to eat blood
QED: The Bible forbids transfusion.
This was the Society's position from the mid 1940's clear up into the late 1960's. It was very clearly set out in the 1944, 1945, 1950 and 1961 Watchtowers. There was actually nothing wrong with the logic here. --It was a valid deductive argument. If the premises were true, then the conclusion would have been true as well.
The problem is that premise 'B' is completely wrong and the conclusion is therefore wrong. The Society abandoned that argument many years ago.
Every single argument that has been offered since then has contained one or more very obvious logical fallacies:
A list of common logical fallacies with explanations is available here: http://www.nizkor.org/features/fallacies/
Here are three common examples:
Equivocation: (Attempting to establish equivalency by generalizing)
A. The Bible forbids eating blood.
B. Eating blood is 'taking in' blood.
C. Transfusion is 'taking in' blood.
--The Bible forbids transfusion
Argument from Silence (Basing an argument on what we don't know instead of what we do know)
A. The only God-authorized uses of blood were ceremonial.
B. God never gave permission to use blood for any other purpose
--Therefore the Bible forbids transfusion
Generalization (Assuming what is true of the sample is true of the whole)
A. Sacrifices made under the Law prefigured the blood of Christ
B. The blood of Christ is sacred
--Therefore all blood is sacred and cannot be used.