I would suggest that the problem is more basic and fundamental than what is being argued.
The Decree doesn't actually say "abstain from blood" --That is not a grammatically complete construction.
What it actually says is, ἀπέχεσθαι...tοῦ αἵματος. -- To keep abstaining from blood or To be abstaining from blood.
It's a clear reference to a prohibition that was already in effect at the time the words were written. (i.e. The eating or possibly drinking of blood)
It therefore becomes incumbent upon the claimant to demonstrate in concrete terms, how and why transfusion would fall under the umbrella of this prohibition. That's not an easy argument to make, as its not supported. by modern medicine.
JW writers attempt to sidestep this responsibility via semantic legerdemain. They invoke an incomplete predicate apart from the context that completes it and pass it off as an independent construction. (I'd be happy to elaborate on the grammatical issue.)
That is not a latitude you should allow. Make them stick to what the Bible actually says.