Plugins do not automatically update themselves. The user receives a message in the Creative Cloud App “Plugin updates available”. The user then needs to click the “Update” button (The “Install/Remove” button becomes an “Update” button).
I’m not 100 % sure, but I would imagine that it doesn’t get uninstalled, but there’s also no way to re-install it => it stays “as long as it can”, but there’s no way for users to re-install it, meaning any product updates to the host app could potentially break the plugin…
I’m not sure how it works with paid plugins, though.
No, users have to manually update them by clicking “Update” in their CCD app (at least that’s been my experience).
I think this is basically the same as in your first question: The plugin will probably remain, but can’t be re-installed in that version.
Sorry, I’m unable to give any clearer responses than that. I’ll try to poke around internally to get this answered, but can’t promise anything