I certainly believe that if the Commander-in-Chief issues an unlawful order, either his next in command will obey that order or not obey that order. If he doesn't obey the lawful order, then he is subject to court-martial just like any soldier.
Then we are right back in the position of having a court-martial to determine whether the offense occurred, a petition to the Military Court of Appeals, followed by an appeal to the Supreme Court of the U.S.
If President Nixon had the audacity to issue the order in my hypothetical, what makes you think that the higher ups in the chain of command won't obey it? Over five dozen people were convicted of felonies during Watergate; most of them lawyers working for the President's administration. Every one of them committed unlawful acts in furtherance of the conspiracy.
“Over five dozen people were convicted of felonies during Watergate; most of them lawyers working for the President’s administration. Every one of them committed unlawful acts in furtherance of the conspiracy.</u>
Were any of these folks members of the military?