Keith will either have the full version done by then, or he’ll release a new beta version with an extended deadline. This has happened many times in the past. Don’t worry about your data. If you want to be paranoid about it, you can always use the File menu/Export Files function the night before, but I’d be shocked if you weren’t already running a new version the night before.
Yup, if Keith decided to be bad and let the licence (or license) expire without another version (either beta or official) he’d have a revolution on his hands.
Having such a faithful group of users also brings duties and Keith has been a great developer regarding them.
Definitely upgrade. A lot of fixes have been made since beta 3. Go to this thread and download it. Once you have beta 5, you can upgrade the app from within Scrivener, too. Makes it a lot easier.