What does a MPI investigator do in the Army reserves?

Probably the same as everyone else. SHAPR, EO, Anti Terror briefs to name a few. Then medical readiness, fill out counseling forms and NCOERs/OERs. Weapon Qual will usually be an extended drill weekend.

You know all that admin stuff Active Duty has to do outside of your job, well that's the majority of the USAR so "on paper" we look the same as an Active Unit.

Your MOS will primarily only be done during Annual Training.

I anticipate when the ACFT is fully active then passing record scores in the USAR are going to go through the floor. So expect PT to start taking a larger portion of drill weekends. In fact BEFORE CV-19 happened PT started being required for all USAR Units within my given 2-Star's command.