Dershowitz is probably wrong on anything related to an “acting” appointment under the Vacancies Act of 1998 to a cabinet position reporting directly to the President. There’s a good chance the Whitaker appointment wouldn’t stand up to constitutional scrutiny here.
Dershowitz disagrees with you, Mark Levin disagrees with you, a lot of other lawyers disagree with you, and the DOJ disagrees with you. Plus I read somewhere that this law had been used over 100 times for just this purpose.
So what do you know that they don’t?
For one thing, I have a strong and powerful legal mind on my side who may very well end up ruling on any court challenge to Whitaker's appointment:
"We cannot cast aside the separation of powers and the Appointments Clause’s important check on executive power for the sake of administrative convenience or efficiency. That the Senate voluntarily relinquished its advice-and-consent power in the [Federal Vacancies Reform Act] does not make this end-run around the Appointments Clause constitutional." -- Justice Clarence Thomas, concurring opinion, NLRB v. SW General, Inc. (2017)
It's important to not that the Vacancies Act may have been used "over 100 times" for Executive Branch appointments, but I'm going to venture to guess that none of them involved the appointment of someone who would be considered a "principal officer" under the Appointment Clause as defined in multiple Supreme Court cases (including the one I cited above).
The bottom line here is this: The U.S. Senate cannot pass a statute that surrenders its constitutional obligation to confirm principal officers in the Executive Branch. If that argument sounds ridiculous, just remember that this is the exact same argument that many legal professionals (maybe even including Mark Levin himself) have made when they object to the "fast-track authority" Congress has given to the Executive Branch to negotiate trade deals.
Joe di Genova, former DC Federal Attorney, disagrees also.
And: If this were Obama, no one would be touching this.