Clearly, the burden of proof is on the claimant to establish himself as a case of (3) rather than (1) or (2). Meeting that burden requires a showing that the allegedly divine directives are consistent with some independent standard of morality.
Thus, the problem is neatly stood upon its head -- humans cannot be reliably guided by divine morality until they figure out the principles of morality for themselves.
Which is impossible, per the ought-is perplex.
Great point.