Thanks for your nice photos, Pete.
These photos introduced in Maru Mechanic show Ki-21 model 1.
In the cockpit of model 1, there was no third seat and Commander, or Captain, sat on the co-pilot seat.
The 3rd seat of model 2 was, of course, for Commander as it was called Commander Seat.
There was no seat behind the pilot seat.
A veteran Ki-21 pilot once said Ki-21 was a modern aircraft and there was even carpet placed on the floor behind the pilot seat.
The Ki-21 was designed to be flown by minimum number of crew of 4 or 5, so each crew had multiple roles like Bomber was also a Radioman and Gunner. Even Commander handled the bomb sight too. Pilot only flew the plane.
Wish above may help.
Shinpachi