Mcrit is just that MN at which a part of the the airflow reaches speed of sound. Cruising Mach usually are above this Mcrit.
The adverse effects of shock waves, tuck under, aileron reversal, buffet etc... occur at faster speeds, after "divergence MN". At this MN, drag increases sharply because the shock waves are very intense.
I guess that if the Mach trim compensator is U/S, MMO will probably be reduced accordingly.