The person who would have to decide if the orders came from the rightful President would be the person who received hte orders from the President.
It’s not the job of the grunts to determine that lawful orders they are given originated with the President. They are only supposed to evaluate whether the orders are to do something that would be lawful.
In this case, the argument is whether the President is entitled to be President.
But it’s no different than asking whether the orders you get really started with an order legally signed by the President. And we can’t have hundreds of thousands of troops each of which can refuse to obey an order until a court decides if the President authorized the order.
A soldier has the right to decide if an order is unlawful. “Go kill innocent civilians for fun!” would be unlawful.
Soldiers do NOT get to decide things like, “Is it a just war?”, or, “Is everyone in my chain of command competent and duly authorized?” Those are required to be accepted on trust.
Believe me - I once tried to turn in my Commanding Officer as a nut job. I was right, but no one would touch it. [Note - in the end, he agreed to retire after some of his comments became too painful for some of his higher ranking fellow officers to ignore...]
There isn't anything lawful about having a Commander-in-Chief who isn't eligible to exercise that command.
Our Founders detested the idea of a standing army. The Militia was expected to ensure that the state was secure. If the Militia takes up arms to oppose a standing army that is commanded by a usurper, doesn't the Militia have an obligation to remove that usurper? How would armed opposition to the Militia by the standing army preserve, support, and defend the Constitution?
To suggest that any order to the army which is not on its face "unlawful" must be obeyed, regardless of who is Commander-in-Chief, is to disregard the oath taken by our soldiers. The oath is to the Constitution, not to the chain-of-command.
I don't see the obligation of the average soldier to judge the authority of his command as being any different than the obligation of a juror to have the final say on the law as well as the facts in a criminal trial.
I never intend to vote for conviction of a person for keeping and bearing arms regardless of the "lawfulness" of a judge's instructions.
I recently recorded the movie "Judgement at Nuremburg" which details the convictions of German judges for their crimes which were, in some cases, completely "legal".
If a court-martial is the expected outcome for a soldier disobeying a "lawful" but unConstitutional order, then that is the course his oath demands.