UPDATE:
The issue (when autostarting the SAAB340) seems to happen during the Left Engine Start verification. Just sits there forever, verifying.
In one test, several errors appeared on the warning display, and the master warning light was triggered.
This time around, I discovered some things:
-The GPU was removed during the process. Viewing the auto-start externally, the GPU is never brought in, but the chocks, etc are removed at the start. I thought maybe this was just a visual thing, but noted that the External Power indication in the overhead panel was off... so the auto-startup was occurring (supposedly) under the plane's internal power... which may explain why I would sometimes get warnings/errors (battery low?... and then other things get affected as a result?)
-The infinite hold/stop that occurs when the Left Engine Start is being verified can be fixed by manually setting the throttle from GRND IDLE (which is what the autostart sets it to) to MIN. This results in the rest of the auto-startup to occur, up until you reach the Right Engine Start verification, whose pause is also then fixed by manually moving the throttle to MIN. The remainder of the auto-startup occurs, and successfully concludes (complete with Auto-Start Complete message). And, the lovely and previously absent Beta engine growl finally occurs.
The only other issue I've noted so far, which I can't find a workaround/fix for, is that the windshield of the SAAB340 in XP12 appears dirty (as if covered by 'calcium stain' water drop marks). It seems begging for the windshield wipers to be used, but unfortunately they have no effect. In XP11, the same installed plane doesn't seem to have this window texture (or it's extremely faint), but instead it has a window scratch texture, which seems to be absent in the same plane in XP12. So, that's a bit of a mystery. The 'raindrop stain' texture appearing in XP12 doesn't seem 'wrong'... it just seems like rain staining after a storm, which could/should be removed by the windshield wipers (but unfortunately isn't). Maybe some 'window dirt' flag/value is (mistakenly) set to high/full on startup, compared to 'clean'. The window scratches, meanwhile, seemingly should be present and at a constant, non-increasing value... but isn't, in XP12. Not seeing any windshield rain effects currently, either... at least with a stationary, unpowered SAAB340.
So, it seems the only issue as far as auto-starting is that (seemingly) the throttles have to be manually set to MIN after each engine start (verification). I'm going to have to try a manual startup again and see if I succeed that way, too.