I'm unclear as to where at least one of the crew members sat in the
Avenger, and whether at least one of them moved around during flight.

The pilot, I'm sure, sat up front and stayed where he was.

Elsewhere in the cockpit, there's a gun turret, and there's apparently
another seat between it and the pilot's seat.

But there's also the "downstairs" space aft of the torpedo/bomb bay,
which has a rear-facing machine gun of its own.

Did the radioman sit down there sometimes and between the pilot and
turret gunner at other times? Did he have room to climb up between the
turret gunner's feet and the rear of the bomb bay?