Fisherman,
In my view, the Decree is exactly what the story represents it to be. --An attempt to mend a rift between the largely Jewish Jerusalem church and the largely Gentile Antioch Church. Although Ancient Greek is rich in imperatives, the Decree is not framed in the language of command and the word of choice among English translators (i.e. Abstain) carries a strong connotation of voluntary compliance, which is reflected in most dictionaries.
This view is neither here nor there as far as JW's are concerned, so I would frame a rebuttal strictly within the framework of what JW's have taught.
It is clear, in retrospect, that they have simply made a mistake. For centuries it was thought that blood was actually a more elemental form of food. This is evident in the writings of Thomas Bartholin, Jean Baptiste Denys and even the historian, H.G. Wells. Consider, for example, a passage from Wells' novel, War of the Worlds:
"Entrails they had none. They did not eat, much less digest. Instead, they took the fresh, living blood of other creatures and injected it into their veins…..The physiological advantages of the practice of injection are undeniable, if one thinks of the tremendous waste of human time and energy occasioned by eating and the digestive process. Our bodies are half made up of glands and tubes and organs, occupied in turning heterogeneous food into blood."
The JW teaching on transfusion was based upon this misconception for at least the first 10 years of its existence. Transfusion was viewed as a form of intravenous feeding that fell directly under the umbrella of biblical prohibitions against eating blood. (The original (brown) edition of the book, Make Sure of all Things states this explicitly as do 50's and early 60's era Watchtowers)
Eventually, JW's realized that blood is human tissue; that transfusion is a form of tissue transplant and that transplantation is both physically and morally distinguishable from consumption. It therefore became necessary to enlarge the scope of the Decree far beyond what they themselves had previously taught. AFAIK, no reason was ever given for this (Other than the fact that it was necessary to prop up the teaching on transfusion)
None of these attempts hold up under scrutiny.