OK, it RUNS!!!!!
I called Doktor A (first time for me... woo). He returned my calls and - he's truly a pleasure to speak to - walked me through pretty much everything I did. He said "don't give up hope. Based on the work (I'd) put in so far, plus the ECM testing good, it was most likely a CAN comm error." Period.
(The other possibilities - a Lack of Power to ECM or a Bad ECU - being eliminated)
And he repeated good advice others had given previously (definitely MWDrifter, Mehr4x4 Eric, Vic, Bill, others?...). Advice I'd heard and followed but not closely enough - that it was MOST LIKELY related to the work I'd done at the ECM connector. As we spoke, it dawned on me that I HAD disassembled the plug further I recalled - more than just removing the outer cover. Behind that cover are two sub-connectors that are very similar in shape. They lock in place within the larger main connector via a nylon collar. Talking to Doktor A, I wondered if it was possible that I'd put them back in reversed... or something. He assured me it was unlikely but to go take a look.
As soon as I finished the call, I checked the van's ECM connectors again. The C2 plug still looked fine from the outside. But THIS TIME I clipped the zip tie that holds the bundle of wires in place, and soon as I did things started to move around in the connector. A lot. Both both sub-connectors could slide in and out freely in the main body. Whut the...?
I knew at a glance that this meant the ECM pins were making NO contact to the pin sockets in the loose connector plugs. Not one of them.
Lo and behold, the sub-connectors had been removed and replaced in the correct order, but both were now resting ABOVE the nylon locking collar. I had simply failed to get the collar reinserted in place correctly. I could only chuckle at this rookie move, and my blockhead attitude as well. In a few seconds, I had reassembled the connector with the locking bar inserted correctly.
The van started normally and ran nicely. Lesson learned.
Thanks AGAIN to every one who took the time to offer advice and encouragement. Mea culpa. I certainly deserve any "I told you so" comments. :-\