> Could he leave the slot open and announce his nominee is now a Special Advisor to him in charge of that department? <
Good question. My layman’s guess is that an Acting Secretary would work, but not a Special Advisor.
And that’s because a Special Advisor has no authority to issue orders to the department. So he certainly would be ignored by the rank-and-file in that department.
All a Special Advisor could do is report every last problem to Trump, and then Trump would have to issue the orders directly. Rather inefficient.
How long can an Acting Secretary legally serve as the head of the department before requiring confirmation?
I wouldn’t take the inefficiency matter seriously; the only problem is that Trump becomes DIRECTLY rather than indirectly responsible for any poor decisions, governing or ethical matters. OTOH, An “acting” secretary usually is simply the number two guy, who still needs to be approved and may have gotten in with less scrutiny. Remember the #2 DOJ official who used his office to persecute Trump instead of prosecuting Obama-Biden-Clinton, once AG Sessions stepped aside?