Does the president have the power to make cross dressing a UCMJ violation and also render prescribed punishment for said offense immediate discharge?
As to yours...I am not a lawyer...but contrary to your worldview, the UCMJ was not created by a dictatorial president, but rather by CONGRESS...as all laws should be. Recently, however, it has been changed by a 'pen and a phone'...ie by executive order. I suppose anything is possible with an executive order, but that action of making law without congress is, IMHO, not in accordance with the constitution and is a practice that should be stopped.
However, I am positive that pre-Barry, cross-dressing was a violation of UCMJ, so any action to now make it illegal would be an undoing of his unconstitutional behavior and would be appropriate...but it will never happen.
Barry didn't feminize the military in one fell swoop on his first day of office. Instead, he waited for 'studies' and 'recommendations' - this is because Barry is not an idiot. For similar reasons, Trump can't undo Barry's work in an instant either.