Thanks Anthony.
I think I've found that the browser is NOT in fact crashing, but that the Exit Activity link is somehow confused and closing the top.window. Scores are being reported back to Moodle.
As to Incomplete -> Complete, vs ->pass, I never liked this silliness about SCORM 1.2. It should be Incomplete / Complete and/or Fail / Pass (or additional states), as I believe it is in SCORM 2004.
So we track pass / fail in Moodle instead of in Captivate, which gives us more flexibility around pass / fail scenarios without having to republish the quiz.