As I mentioned, not sure if it's a requirement Thailand ask to go back to the realm. It's for sure a requirement inside Thailand when your old passport is cancelled and you got a new one, you must transfer your immigration files to your new passport.
That's how it works with US visas, but not sure for Thailand. Most countries of the world want you to transfer long validity visas to your new passport.
Embassy is the only office outside of Thailand able to put the re-entry stamp to the new passport or give you the correct guidelines. Call them and ask for the procedure.
oh, this is not a translation issue but rather a cultural one. The sister can't come, problem solved. The Thai won't understand that you bother yourself with plans that were changed/cancelled. In your case, a Thai would have asked "She can't come, at what time do we go then?"
Everybody can renew their driving license all around Thailand, Thai and foreigners alike, wherever their address is. Many Thai don't live at their registered address and transport department decided to take that into account some years ago, thank to their internet linked system.