Took a chunk of my tax return and had the Blue Lemon towed (with a freshly charged battery) to a local mechanic I know and trust as being a strictly honest fellow, and who relishes electrical challenges. Gave him the full story and let him have a crack at it.
The next day, after going through multiple 10 amp engine fuses (apparently they would only blow as he turned the key off after cranking?) and getting some really oddly low RPM readings he found the Root Cause:
The harmonic balancer/toothed crank position sensor trigger wheel/crank pulley unit was disintegrated. The toothed wheel that the crank position sensor reads is bonded into place with rubber, and this rubber was rotted out with heat and age.
The oddly low RPM readings were the toothed wheel hanging loose and spinning only partially with the crankshaft. Sometimes it would hang up on the destroyed plastic of the lower timing belt cover and not spin at all - and with no crank timing input from the sensor, the ECM would not send spark.
One brand-new balancer unit later, car starts and runs fine. It doesn't even blow the 10amp engine fuse any more.
Now I have a spare car, as my new daily is an amazeballs clean low-mileage B16 Sentra SER Spec-V with 200hp and a 6-speed manual. I will continue to swap good parts onto the Blue Lemon from my parts car (especially interior) and eventually plasti-dip it before my 13 year old son needs a car. I also plan a full timing belt job with new covers, LOL.
Lesson learned: crank but no spark: check the crank trigger wheel for wiggle.