First, off mayyeejeh welcome to Seek Adventure! Let me try and answer a few of your questions.
mayyeejeh -Trip date: early December. Yea, I understand that chance of seeing stars are probably lower in this season, but crossing-fingers.
Depends how far away you live, are you flying or driving? I ask this because it would be good to check the weather forecast, stars can be hit or miss depending on the sea haze that rolls in.
I did not check the weather too much before I went but I had some sea haze roll in early in the evening, so the stars were covered at first but the haze created an amazing sunset for me!
You can check out the coastal weather here when planning your trip: National Weather Service - Big Sur Coast
The second thing to look at when wanting to see stars is what the moon is doing. If you have a full moon then the stars can be washed out. I checked the moon schedule for you and from 2-8 December 2018 the moon will not be a problem for you:
So as long as the weather works out for you, then you should not have a problem.
mayyeejeh Do you think it's safe coming up to/leaving the campsite during night time with a sedan? (most likely not 4-wheel drive)
The road was not that bad when I drove up, and I saw some two-wheel drive small sedans during the day.
I will caution you if it rains right before you go, that you should definitely not go in a two-wheel drive car, and make sure your car has bright headlights. If you have crappy headlights I would not go.
With all that said, conditions are always changing, so you never know if there is a down tree or if some old rains washed out part of the road, so drive slowly and turn back if it gets hairy. Do not try and push it in a two-wheel drive car.
Let me know if you have any more questions, and again welcome to the forum, hope to see you around more!