Is Whitaker's appointment Constitutional?

Not according to this analysis.

"While the FVRA allows the president to appoint another Senate-confirmed official to fill a vacancy, here the president has elected to rely on another FVRA provision that allows him to appoint a senior Department of Justice official who was not Senate-confirmed. There remains an open question of whether it is constitutional to rely on of the FVRA to appoint an official not serving in a Senate-confirmed position to act as a principal officer, such as the attorney general."

He also notes that Supreme Corp Judge Thomas has previously arugued that "that an acting principal officer must be appointed in conformance with the Appointments Clause, i.e., by and with the advice and consent of the Senate." Let's see if Thomas holds to this argument when this is challenged before the Supremes. Being the partisan hack he is something tells me he'll change his tune.