The Mueller probe is illegal BECAUSE it is unconstitutional. I see nowhere in the Constitution the power to create a “special investigator”, especially
with no probable cause to investigate.
It's really no different than a police chief removing himself from a criminal investigation when a crime victim or a suspect is a family member. Appointing an "acting chief" to oversee the matter doesn't violate anyone's constitutional rights.
The Justice Dept. and the FBI are not in the Constitution either. That doesn't make them unconstitutional.