Can he put a Supreme Court justice as a recess appointment !!! Is that legal??? Not that legal matters with this crew!!!
Sure. It's been done. The only thing stopping him is convention.
Eisenhower recess appointed Brennan and Stewart, and elevated Warren to Chief Justice on a "recess basis." George Washington made a SCOTUS appointment on a recess basis too, but his pick was not later confirmed.
Who would ultimately decide the ‘legality’ of such an appointment? Do you have the slightest confidence that SCOTUS would reject such an appointment? Or that Congress would act?