Q.: What kind of scandal would cause most JW to leave the "truth"?
A.: I note that you specify "most JWs" in your question. For an answer, I think we need to look at WATCHTOWER history and look to see what caused massive, large-scale defections in the past. Fortunately, WATCHTOWER history offers plenty of these kinds of incidents to examine.
"New Light" will never do it. Yes, SOME will leave as a result of the cognitive dissonance caused by some "New Light," but that will never be cause enough for "most JWs" to pack it in. Look at what a relatively small impact the UN scandal caused. Most JWs just shrug that off as a lie even though if they call the Bethel Service Desk, the Service Desk will CONFIRM that it happened.
So, to cut to the chase, I think the answer lies in one word: RUTHERFORD.
The GB are mortal men and like the Mickey Mouse Club, they have to bring in a new member from time to time. IF one of these future GB members proves to be a NEO-RUTHERFORD -- a self-centered sociopathic power-seeker -- THAT could be straw that breaks the camel's back for "most JWs."
Certainly WE of the Apostate Legion have no control over such an ascension. The next beast in the image of Judge Rutherford must arise from the chaotic 'sea' of official Dubdom, an organization by and for MEN. We need only remain vigilant, patiently waiting and watching for that Great Day.
When it happens, the WATCHTOWER will be shattered into hundreds if not thousands of fragments, many being, as there have been today, small home-based groups, but there will also be millions worldwide who will be wondering what to do now. Some will fall prey to almost anyone low enough to offer everlasting life in exchange for submission. Many others will try to make sense of the "new world" they now find themselves in.
That's what I think, and I'm sticking to it until I change my mind.