Well I feel slightly stupider than usual. I had convinced myself I was overcomplicating things so I thought I would regroup. Come to find out the hopper motor that runs at less than half an amp on the bench had a huge initial startup current (over 7 amps). That momentary startup current exceeds the overcurrent protection limits of the A3952SW drive IC and caused it to shut down.
I ran the motor for a couple of minutes on the bench. The startup current went way down as the motor warmed up, so I put it back in the game. Low and behold it was working.
Now where to find a motor for a S-2000 hopper? Can you get just the motor? Motor and gearbox maybe? I don’t think it would be practical to ship an entire hopper for the sake of just a motor.