Played a bit more this morning (once I had dealt with the latest bits of sky falling on the Nexus) and learned a bit more about the "locking" quest scripts.
Clean saved NCCS from my game and re-enabled it, and the main quest was dead on activation. Never functioned at all this time. This tells me that the problem is something fundamental in the game itself -- whatever it is, the game has decided the script does not need to work under any circumstances. There's no fix, except to revert to a save before it happened, or start a new game.
FONV continues to be impressive in its unreliability. I'm at 65 hours or so now, and crashes are frequent; but not quite constant. Most often is a crash on changing cells, whether it be by door or fast travel, it tends to lock on loading the new area.