Just note that on Windows and the Mac, it does not delete the local databases those apps have. So while you cannot launch Evernote.exe and use it, both apps keep their data on your PC is pretty much plain text format, unless you have specifically encrypted text. Evernote relies on your PC's security model for access to its local database, so passwords, encrypted hard drives, etc.
On mobile devices it pretty much does block access and all iOS devices and most high end Android devices support
Yes. It simply blocks further syncing. If the client is running, it will not force-close the client. You are revoking access to cloud data, not initiating a logoff or a wipe-data command. This is not designed as a security tool. It is simply a way to control your device count. Free users only get access to two clients, and this provides them a way to revoke unused clients so they can get another client working.