In an interesting brief filed recently, people that know the law a lot better than Merrick Garland think Jack Smith's appointment is illegitimate and therefore his charges against Trump must be dropped. Jack Smith was not properly appointed.
https://reason.com/volokh/2023/12/20/special-counsel-jack-smmiths-appointment-is-unconstitutional/The brief outlines four reasons Jack Smith's appointment is unconstitutional. If Smith continues to go after Trump, any result in an inferior court could be fought simply on constitutional grounds at the Supreme court.
First, all federal offices must be "established by Law," and there is no statute authorizing such an office in the DOJ now that the Ethics in Government Act has sunsetted out of existence in 1999.
Second, even if one chooses to overlook the absence of statutory authority for the office, which Jack Smith currently holds, there is also no statute specifically vesting in the Attorney General the power to appoint an inferior officer Special Counsel with the powers that Jack Smith currently has of a Senate-confirmed officer.
Third, the Supreme Court has defined an inferior officer as being one who is directed, supervised, and controlled by a superior officer in Edmond v. United States, 520 U.S. 651 (1997) and in Free Enterprise Fund v. Public Company Accounting Oversight Board, 561 U.S. 477 (2010). Attorney General Merrick Garland is not, and cannot under the DOJ regulation under which Smith was appointed, direct and control Jack Smith's activities in a way that satisfies the Edmond test.
Fourth, the Special Counsel is, in all events, a superior, like a U.S. Attorney, rather than an inferior officer, like an Assistant U.S. Attorney, and thus cannot be appointed by any means other than p**********l appointment and senatorial confirmation regardless of what any statutes purport to say.