I would agree with that, but only as long as Whitaker's functions are entirely administrative in nature. The moment he gets involved in a matter that requires the U.S. Attorney General to sign off on it by law, he's going to have a problem.
A Federal criminal prosecution in a death penalty case, for example, requires the approval of the U.S. Attorney General. If Whitaker were to sign off on one of those, the defendant in the case would absolutely win an appeal based on the unconstitutional appointment of Whitaker.
This is why I suspect Whitaker will do nothing in any "official" capacity as the AG.
You will lose this argument, for a host of reasons. For one, it has already been decided, against you. Presidential temporary appointments under the subject law have been upheld. Doesn’t matter WHAT the caretaker does; only that someone has to be in charge, and that person is. That person has to have been already approved by the Senate for that agency, and the length of time of the appointment is limited. Seems a very reasonable method of allowing a succession to take place in case of resignation, firing, death, etc. Congress takes months to confirm a successor; a system is needed. Next man up is not necessarily the best way to do it. Luckily, Congress passed a reasonable solution.