kyangareth: Yes, in this conception, the Working Group would declare a given property’s implementation to be interoperable and thus able to drop the prefix. Upon doing so, it would also formally mark that property as being interoperable. (And if there were only one implementation of a given prefixed property, it would stay prefixed forever.) The method the WG uses to reach that decision would be up to it, but I would strongly favor test suites that any implementor could run through to show interoperability. Whether they’re automated or manual doesn’t really concern me; either is fine, though automated is obviously less work to evaluate at the cost of (marginally?) more work to construct.
To the second part, what I envision is that any module which seeks to reach CR stage needs to have all of its constituent properties exist somewhere in an unprefixed state. I don’t think they would have to be unprefixed in all the same browsers, although I wouldn’t oppose that requirement either. Basically, any property which has no unprefixed implementations would be enough to keep a module out of CR. Either it would have to be dropped or interoperably implemented.