Instead of a Mr. Buddy heater, I would recommend (albeit this is more expensive) a battery + 12v heated blanket. IMO this is less nerve-racking and more safe.

While I agree that a heated blanket is the superior way of keeping warm in a car vs an electric space heater, every single 12v heated blanket on amazon has reviews/pics of the blankets shorting out and melting/smoking. Even if you are careful, it seems like it's only a matter of time before the blanket gets used so much that some wiring somewhere degrades.

Therefore, if safety is your top concern, why not run a tiny electric space heater if you've got the power (1000W power station). You can buy ones with tip over sensors and thermostats. Keep in mind this would be in addition to all the other methods you mentioned like thick pad, extra down blanket over sleeping bag, etc).