3. It was a capital offense to use blood for any purpose other than to offer it as a sacrifice on the altar.
This is actually not entirely correct. You forgot another permissible use of blood outside of the sacrificial process - a use of blood that exposes the JWs' ban on blood transfusions as being illogical. I'm talking about the primary function of blood - the function for which it was created by Jehovah. The prohibition on blood obviously does not apply to our use of our own blood in our veins.
So there are actually two permissible uses of blood: (1) on the altar (explicitly stated in scripture) and (2) in our veins (not explicitly stated in scripture but obviously implied by the fact that the Law did not instruct or require the Israelites to slit their throats and bleed themselves to death).
Do not quickly ignore this second, permissible use of blood because it has a direct bearing on the blood transfusion issue!
Given that using our own blood in our veins does not constitute a violation of the command to abstain from blood; and given further, that the command to abstain from blood says nothing about the source of the blood ("any kind of blood", to wit JWs aren't to be transfused even with their own pre-donated blood); we can logically deduce that using donated blood in our veins would also be permissible.Why?
Transfused blood is used by the body in the same way and for the same purpose as native blood. Thus we can see logically that the command to abstain from blood - really, abstain from the eating of blood - cannot be applied to transfusions since transfusions involve use blood for a permissible purpose - the same purpose we use our own native blood.
Eating blood is different by virtue of the fact that it involves equating the soul with food, which has the effect of undermining blood's spiritual value to atone for the life of the sinner on the altar. For "the soul is worth more than food". (Matthew 6:25b)