Solved! Posting here in case anyone might get tripped up by something vaguely diabolical but simple, like this:
Sean at eSim thought to try once more without Avast AV, despite the fact that it has been running along with SBP for the past year or two, and the fact that SBP runs on my old computer, also with Avast AV, and doesn't CTD. But that's what it was.
I can't get Avast to launch its UI in the background at boot, although the service is running and doing its watchdog thing. Like Real Time scanning. But apparently, no warnings or termination reports are triggered without that UI and its little icon displaying in the lint tray on the desktop. When I disabled real time scanning, SBP ran. I made an exception for SBP, re-enabled the scanners, and SBP continues to run - and defaulted to the windowed mode as you said, and unlike the several installs I made over the past week.
Then why didn't SBP get crunched for the last year by Avast, regardless of the issue with pop up warnings not displaying? SBP was very recently patched up from the major expansion of this cycle. And Avast auto updates its virus definitions each week. These two things collided - silently - this week.
Why didn't my other install of SBP - on my wife's computer that also runs Avast - also CTD? It's an older version of SBP I stopped using quite a while ago when I gave that machine to her. That SBP executable and the current Avast definitions don't clash, so no pop up to let me know that was the cause.
I apologize for taking up so much of your time on what turned out to be a pretty simple solution! Kind of a weird combination of events colliding to create a fog of war... But thanks for your patience and assistance.
ASUS P8Z68-V PRO
BFG N460 GTX Cyclone 1GD5 OC
X-Fi Xtreme Gamer
2 gig RAM
Saitek X52 PRO.