Re cores, doesn't a nationalist revolt go off without cores and end up spawning the nation that is considered to be the primary tag for that culture?
IE, if Scotland's cores disappeared, they could still come back via revolt but not by way of another country forcing their release. Hence why PDS removed the primary tag settings for Burgundy and the other French duchies but not for Brittany. Now, the fact that they don't expose this in the UI is a bit annoying, and the fact that they apparently have different codepaths for almost every way a province can change hands so any change they try to make ends up only half implemented.
EDIT: Ooops, Scotland is an accepted culture of GB so it probably wouldn't nationalist revolt no matter what. That said, Welsh is not, so it's a better example. Current system won't eat primary tags for cultures within the game timeframe if it doesn't exist, mind, but I'm talking about the original design of the 1.3 system.
That said, it is pretty hard post-1.1 to end up getting nationalist rebels and even more difficult to get them to win. Also a problem is that there's no distinction between homogenization of culture (ie, the Revolutionary French getting rid of the regional variations or even different cultures depending on some accounts as the idea of the 'Nation of France' instead of the 'Throne of France' was now their unifying characteristic and as such they couldn't stand anyone that didn't consider themselves to be French first), the lightly populated areas being repopulated (North America, Horde regions by Russia, etc), or forced migrations/etc (some Balkan areas, not sure on a few others). Or the revival and/or creation of cultures (for example, Finnish culture re-established itself after being colonized by the Swedes then conquered by the Russians, and the Bosniaks that are depending on which historical narrative you follow a Islamicized Croatian culture.) So it is still a bit easy for tags to entirely disappear because their cores _and_ their culture have both disappeared, but that's a different issue than the core timer.
Thanks,
-Atma