Ha ha, of course I'm overlooking something simple and stupid, I'm convinced of that now that we've walked down every other avenue. And after all this has been cleared up, I'll come back here blushing like a debutante at the ball, crying shamefacedly "Now why didn't I think of that?" (or you, for that matter). Noob is a relative term.
As a longtime user of BIAB I'm as surprised as you are by all this. However, support has offered a remote session, so they can look under the hood. I'll keep you posted (if I'm not too embarrassed).