Hi Sam,
1. We made the off-pavement part of our trip in about an hour and a half. But in a sedan and allowing for the road to develope "problems" I would allow two and a half hours.
2. You can go to http://www.nps.gov/deva/Morning.pdf. The report is updated just about every day. This lists the weather forecast (except Death Valley is a really diverse area so I'm not sure what elevation/location the report should "acurately" portray ... maybe Furnace Creek?). It also lists temperatures for the last 24 hours and road conditions. The one you will be most interested in is Saline Valley South.
The problem with trying to get a forecast is that nobody actually lives/has a weather station where you are going. To say the least, weather forecasts turn out vague!
3. Yes, it is possible to drive on the road at night. You see vehicles showing up at all hours of the night. Although it will be more difficult finding the correct turn-off (particularly for the spur road off of Saline Valley Road) if you have never been there. That could be handled with GPS coordinates, owever.
You could drive out for just a day-visit but I'd never do that. Once you've "earned" your soak (dealing with the distance and road) you won't want to leave anytime soon!
You are correct assuming there will be lots of vehicles out there that weekend. And if you've done any off-roading, you know that these people are very helpful and friendly. They will never leave you stuck!
Hope you can make it.
PS The water there is drinkable, there's even showers (where else can you clean up and enjoy the scenery??) and kitchen sinks!
Here's some pictures:
http://www.geocities.com/hotspringer2000/nature_lover2.html
http://www.geocities.com/hotspringer2000/nature_lover3.html
http://www.fortunecity.com/healthclub/stretch/229/nature_lovers4.html