The supposed "investigator" is not outside the DOJ. The investigator has worked for the DOJ for a long time. The investigator is "outside" of DC.
Since Sessions is recused on these matters, this investigator would answer to Rosenstein.
The Russia collision and DOJ/FBI abuses are two separate matters. One deals with investigating outside foreign collusion while the other deals with potential DOJ/FBI violations of Federal law and abuse of power.
Sessions is obligated to investigate abuse/violations within DOJ/FBI regardless of Rosenstein or his recusal of the Russia matter. What Sessions is doing in an internal AG matter.
Mueller is free to investigate but the DOJ/FBI can’t violate the law in the pursuit of their duties.