It would be vaguely interesting to know from study participants how unique their browser is.[1] Mine appeared to yield the same result whether in incognito mode or not, but I can't really see Google using this approach.

Your point about Google trends is counterintuitive but evidence-supported, which makes it particularly interesting. It may well be that they are looking at outcome measures after search personalization, and abandoning personalizations that yield no results. Or perhaps a certain volume is required over time for a particular query before triggering an automated personalization trial. As you say, it's a black box, so who knows.

[1] = https://panopticlick.eff.org/