IT WORKS
I did three more things after using IMD to adjust the RPM to +/- 300:
1) Backed off the screws holding the upper head assembly, and pushed it as far back as I could, as I could see there was a tiny bit of play there.
2) Torqued the head (gently) to the left. It had been torqued too far to the right originally.
3) Did as suggested and with the disk out very carefully eyeballed the two heads into visual alignment with some left/right lateral adjustment. It's very hard to do because the spindle is in your line of vision, but I got an angle that worked.
I fired up, bang! Reads disks from other drives, writes them, other drives read what it wrote ok! Initially I didn't think it was working, but apparently the MPI drive is much quieter than the Tandon.
First time in probably decades that drive has worked! Thanks so much to everyone, as usual, for making suggestions.