Zeus, I think that the Panda rating of existing sites would be reset once all their Google traffic has totally gone for a long enough period, say 1-2 months, and the site would become eligible for traffic again, like a new site (assuming they were only affected by Panda). Like a kind of extreme version of the noindex solution many have claimed helped their sites recover partially while they improved the quality of those sections of their sites.
However, if the site quality hasn't improved when traffic returns then the site will just be demoted again.
Regarding where the metrics come from, I still haven't seen a quote from Google that says Chrome data isn't used for Panda (I've seen a quote that says data from Chrome is not used in the main algo, but that's not the same as saying it's not used for Panda).
It's definitely not Google Analytics though. The data in there is too inaccurate (even Google have said GA data should be used as a guide, not relied on 100%). Chrome data though - that would be very accurate.