After five minutes of deep thought, I've opted for dogged perseverance. I'm going to try yet again to fix this trusty grinder. If the alignment problem is solved, I will then buy new burrs.
So I will then have a great grinder with 59 mm burrs, spinning at 1300 rpm driven by a 445 watt motor, all for under $50...versus several hundred $ to a thousand for a new grinder.
At the moment I have tools and grinder bits all over the sink and two kitchen benches.
From the McIver school of invention I made a "dial gauge" to check runout. It's a steel ruler which straddles the top of the grinder chamber, with a small skewer at right angles to it and held on by a small g-clamp. I set the skewer depth until it just touched the wonky sweeper plate.
Gently turning the sweeper by hand confirmed there is a high side. Now, I would have thought that the 30 mm x 6 mm washer on top of the plate would would iron out any minuscule deflection when the nut on the motor shaft is tightened against it and the burr. Not so.
So I will have to experiment here, perhaps some paper shims on one side of the stationary adjustable burr? We'll see.