You did not tell me much about the 3-blade impeller you are using, but almost any properly sized marine propeller or hydrofoil impeller should give you good solids suspension.  A 1/4 hp mixer running at 1750 rpm should have a 3.5" or 4" diameter hydrofoil impeller for best results.  You should be able to feed the mixer shaft from inside the bung hole and into the mixer drive before replacing the drum lid.  Trying to fit anything other than a collapsible impeller through a 2" opening is not likely to give you much mixing.  I would suggest a 1/4 hp electric motor instead of the air motor, unless you are in a Div. 1 hazard situation.  The electric motor will be quieter and more efficient.

Unless your benzoyl peroxide particles tend to stick together or dissolve slowly, a good mixer should take no more than five (5) minutes to uniformly suspend solids in a 55 gallon drum.  Actually, suspension should occur in less than 30 seconds, but just for good measure try five (5) minutes.