Yes it is illegal. NARA by law is required to get the presidential records for the presidential library. Also Presidents lose their security clearance after leaving office.
The Presidential Records Act (PRA) of 1978, as amended, 44 U.S.C. §§ 2201-2209, governs the official records of Presidents and Vice Presidents created on or received after January 20, 1981.
Yes the current administration can grant security clearance to ex presidents. When Trump is no longer the president. He cannot decided anything about classified records.
No one is above the law has to mean something.