Having done the Death Valley Thing a few times, I enjoyed your report and then thought I'd chime in with some of my experiences.
(Some photos are here: http://gdanmitchell.com/gallery/v/Landscap...rt/DeathValley/
I've been meaning to go there in December/January, but my visits have all been during the very first week of April. I've encountered weather ranging from in the 90s to near 100 right on down to sub-freezing with snow. I've been rained on there at least three times.
The scale of DV is huge. For those who have shot other NPs, this comes as something of a surprise. The first time I went there I sort of thought "I'll shoot here in the morning, then go over there, and after that I'll hit the other place, and then..."
Really photographing two locations a day is generally about it. In some cases you might get a third it but only if you plan and time things carefully.
I have not been up to Eureka Dunes yet. In a way, I haven't been able to get charged up about shooting dunes. I have shot the dunes at Stovepipe Wells a couple times, including at night. I'm sure I'll get to Eureka one of these times.
I've been to the Racetrack Playa a number of times. It is my favorite place in DV - not just for the photography, and not just for the moving rocks, but also for the beautiful and rather extreme isolation of the place. There is nothing quite like walking across the vast expanse of that playa - perhaps under a full moon and with the wind blowing.
My reaction to the road is perhaps a bit different that the OP. I have driven it in a AWD Dodge Durango and a AWD Subaru Outback. Both handled it fine, but it is not a fun road to drive at all. I think that the park service may possibly allow this road to remain in fairly horrible condition partially as a way of ensuring that too many people don't attempt the drive. The road is not steep at all, so AWD doesn't really seem necessary (though rain/snow would definitely change that) though reasonably high clearance is important.
The road is generally well graded but terribly washboarded. I've heard of (and briefly tried) the "go faster than 30mph and it smoothes out" approach, but there are just too many ways for things to wrong quickly and badly on this road at those speeds. So I tend to go slower... and get the crap bounced out of me for an hour and a half each way. Ah, well.
I would not bother to drive out there for a daytime shoot. In my view, the _only_ way to experience the Racetrack is to stay overnight - during a full moon if at all possible. There are no services there whatsoever and no water is available. I've seen as many as perhaps 20 other folks out there, though one time I only ran into one other person.
I like the OP's idea about photographing Mosaic Canyon (and potentially other areas) at night with artificial light. I've done some night photography at the Racetrack but not with any artificial light. Oddly, I found Mosaic Canyon and some of the other canyons (Golden Canyon, for example) to be worthy of photography during the midday in the winter season. Something to think about if the weather isn't too darn hot.
I've written enough for now... so one more photo from the Racetrack.