Thanks for the citation. There's no language in that law that restricts the President's ability to directly appoint members to the military who are loyal to him. All it does is establishes a commission, chosen by the President, that can appoint members to the military. But it does not restrict the President from doing so. In fact the President is free to fire them all and let it languish.

Can you cite any other law or precedent that would restrict him from doing so?

Donald Trump doesn't give a fuck about any civil service act. He never has, and never will.

He will do as he pleases, and a GOP led congress will simply let him, laws and precedent be damned (and the Supreme Court will comply). A DEM led congress will throw up their hands in uproar, and then refuse to actually do anything.