Scenario 2: The Federal Vacancies Reform Act of 1998
A more complicated scenario would be to rely on the provision codified at 5 U.S. Code Section 3345(a)(2), which states: “notwithstanding [the default rule that the deputy becomes the acting], the President (and only the President) may direct a person who serves in an office for which appointment is required to be made by the President, by and with the advice and consent of the Senate, to perform the functions and duties of the vacant office temporarily in an acting capacity subject to the time limitations of section 3346.”
In other words, the president may direct anyone who holds a “PAS” office—one requiring presidential appointment and Senate confirmation—to serve as acting attorney general for 210 days (and potentially longer, depending upon when the president nominates someone to the position). And under the terms of the Federal Vacancies Reform Act, the individual does not even have to be serving in the Justice Department at the time he or she is tapped to be acting attorney general; this person would just have to hold a Senate-confirmed position.
http://www.slate.com/articles/news_and_politics/jurisprudence/2017/07/if_donald_trump_fires_jeff_sessions_here_s_how_he_ll_appoint_a_successor.html
