A bad 12V battery on an EV shouldn’t kill the car if it’s already running. It’s just like an ICE where it’s needed to start the car and then the alternator/traction battery is supposed to charge the battery and maintain a voltage for the 12V system. In an ICE, it turns the starter. In an EV, it engages the contactors for the traction battery.
I did have a car die once while driving. If I turned on the headlights it would die. That’s because the alternator was bad and not charging the battery.
My Bolt has a resistive heater like you’d plug in to warm up a room. I’ve watched it use energy via Torque and I’ve seen it up to 3.5 or 4 kW. I preheat my car while it’s plugged in before I leave for work. It uses the heater settings it had when I last turned the car off.
Other cars have a heat pump and use waste heat from other systems.
With the temperature at like 45F the other day, my Bolt’s transmission coolant reached about 110F while driving and the battery was in the 40s-50s. It would be a pretty slow warm up if all I had was a heat pump (but I just have the resistive heater instead).