Trump stated he wants to administer the insurrection act which federalizes the national army.
The 'national army' is already federalized, the Insurrection Act: 'The Insurrection Act of 1807 is a United States federal law (10 U.S.C. §§ 251–255; prior to 2016, 10 U.S.C. §§ 331–335; amended 2006, 2007) that empowers the President of the United States to deploy U.S. military and federalized National Guard troops within the United States in particular circumstances, such as to suppress civil disorder, insurrection and rebellion.'
Active/Reserves on AD are Title 10, normally precluded from this work under Posse Comitatus and the Navy/USMC by other statutes. Federalized NG are also Title 10 and answerable to the National Command Authority (NCA) not their Governor at that point.
Nothing in Gen Milley's letter is suspect. Esper OTOH IMO is an idiot (unless an act). Nobody should care what the SecDef has to say when in conflict with the President. His counsel on the Insurrection Act or any future/possible action by DoD belongs in a Cabinet meeting. He's been around long enough to know that. If he is Derp, he is subject to the UCMJ. Some possible charges:
77 - Principals: Association - Article 77 does not define an offense. Its purpose is to make clear that a person need not personally perform the acts necessary to constitute an offense to be guilty of it
81 Conspiracy
88 Contempt towards officials
94 Mutiny sedition
99 Misbehavior before the enemy
104 Aiding the enemy
And based on actions maybe 112 Drunk on Duty