I don't disagree with your analysis, but I think the problem with the JW interpretation of the Decree is much more basic than that
What the Law forbade was the eating of blood. Transfusion would fall under the umbrella of this command only if it can be shown to be either physically, morally, ontologically or in some other way equivalent to the eating of blood.
It can't and the JW's know it.
They therefore attempt to get around that inconvenient fact by invoking the phrase, "Abstain from blood" as in independent construction in an attempt to convey the idea of a simple and direct command applicable to all uses of blood
That is semantic legerdemain.
You can't invoke a partial predicate apart from the context that completes it. It's ungrammatical.
Allowing the JW's to get away with such a cheap trick is a level of charity they really don't deserve.
"Abstain" is intransitive, which means it is unable to take a direct object or transfer action from subject to object.
What, for example would it mean to:
Abstain from shrubbery
Abstain from crankshaft
Abstain from train
These phrases are completely nonsensical because they're grammatically incomplete
The only way to make an "Abstain from" construction work is to either preface it with an appropriate context or modify the intransitive verb, "abstain," with a finite verb. Only then do you have a transfer of action between subject and object.
Like this:
Abstain from cutting the shrubbery
Abstain from scratching the crankshaft
Abstain from riding on the train
Another way to illustrate this problem is to attempt to express the abstention from blood as a, "Do not."
For example, "Fornication," in contrast to the other three items listed in the decree is the name of a finite act and therefore has a verb form.
I can easily state that abstention as a finite negative:
Do not fornicate
You can't do this with blood because it is not a finite act. It's an object and there is no verb form of the word that really works here. The only way to phrase the abstention as a "Do not" is as I've explained above.
This might seem like an esoteric point, but it's really not. It's actually very easy
In context, the phrase "Keep abstaining....from blood" is a reiteration of the prohibition on eating it as stated in the Law. That's why the word, "Keep" (Which the JW's like to omit) is there.
Out of context it is an incomplete thought that can mean anything a snake oil salesman wants it to mean.
---One more simple example and I'll shut up.
To illustrate how an "Abstain from" construction will morph depending upon the context in which it is spoken consider these two sentences:
His dermatologist said, "Persons with sensitive skin should abstain from alcohol."
Her obstetrician said, "Pregnant women should abstain from alcohol."
Even though both doctors have said, "Abstain from alcohol" they are clearly not talking about the same thing. While we would understand the former to be a reference to the topical application of alcohol, we would understand the latter to be a reference to its consumption.
The two abstentions are completely unrelated as they are meant to address two entirely different conditions.