[rxsid]

So far as I can tell, there is nothing in any of those that pertains to being a plaintiff in a lawsuit questioning the Constitutionality of the POTUS.
Perhaps you could point to a specific provision or statute or law?

[SandRat]

For starters the use of his military rank.

He must file individually in a suit not as part of an identifiable political action.

There are words used in the “letter” that disparage the Pres. and that's an offense covered under the UCMJ.

There is also conduct unbecoming of an officer as well as conduct detrimental to the good order and discipline of the military both UCMJ offenses.