Great but frustrating news!!
The car is alive. It runs. The problem appears to be fixed.
I had been coming across an old, apparently discontinued, part number for the crankshaft position sensor. It is NLA. But I found one on ebay, bought it, and now the car runs. This has been almost six months chasing this no-start and throwing two new, supposedly correct, crank sensors at it. So incredibly frustrating, but thank goodness its alive.
The part number that works on my engine, with a 91 318i ecu, is 12141721968
The one that didn't work was 12141721504