I've been waiting for someone to pick up on it, but I guess ya'll just ain't gonna.
If the president passes down orders to anyone other than his immediate subordinate in the chain of command, he is then in violation of the chain of command. The chain of command works both ways. Hence, it's impossible for the president to give lawful orders directly to field units, because it would be a violation of the chain of command and therefore, would be an unlawful order.
Goddamn, I can't believe I had to actually explain to ya'll.
Now, ya'll are more than welcome to show me the law, or regulation that allows the president to circumvent the chain of command.