I would imagine one NPC controls the car for the entire game.

I've had a spectral cat a couple of times, you can play with it like a normal cat. Not sure if It was in my witches relationship panel... can't really remember.

But if you have two cars on one lot, would that not force two NPCs, like when you place two coffee bars on a community lot? Or am I misunderstanding how invisible NPCs work?

There are other forms of corruption as well. My daughter and I both had them. Another form is disappearing Sims. A Sim that was in your neighborhood and appeared to be perfectly fine disappears. Entirely. Any reference to that Sim appears as $Subject. You can't bring this Sim back, since there is no Sim to bring back. If you or the game generates a new Sim, that Sim slips into the slot held by the disappearing Sim. This form of corruption you cannot fix. Even if you manage to prune the memory of your Sim from every user-made, playable or non-playable character in your hood, the game keeps deleting Sims indiscriminately.

You could go to a backup hood, but there isn't any way to keep it from happening again.

Oh, and the game does it more than once. Unexpectedly. I had several Sims disappear before I had to junk the hood.

It's helpful to read this thread. I have had the annoying problem of Sims bringing home an Ottomas from work. The Ottomas clan is still in my N.H. family bin. First the dad, and yesterday his wife.