“The shedding of innocent blood can’t be both Divinely forbidden as an abomination, and Divinely commanded as obligatory.”
This is where I have to disagree. God can forbid a thing generally, but still command it in specific instances. A good example of that is the law against idolatry. God forbade the making of any graven images generally, but he also commanded the Israels to make several graven images specifically, like the cover for the ark, the bronze serpent, and the adornments for the temple. I would liken this to a parent who has a general rule “No television after 10 PM”, but who still reserves the ability to make an exception to that rule for special circumstances.
If we admit that God, in His sovereignty, can command such exceptions, then the difficulties with these cases vanish. Men are still bound by the commandments, for we cannot make an exception to a rule that we didn’t institute. Only God has that ability, and He seems to use it very rarely from what we see in the Bible.
Assuming that when ordered to commit what God calls an "abomination," you can distinguish between the Voice of God, schizophrenic psychosis, and the voice of a Fiend from Hell.