The first thing you need to figure into your calculation is total length of time your trip is going to take. I don't see that so forgive me if I missed it. If you're hauling butt and make the trip in 20 hours then you have 86400 seconds of driving time. At ten second intervals this would give you 8640 exposures which equals 6 mins of footage (1 frame=1 exposure).

*special note I like your decision to go with 24/fps, looks better.

Whether or not you have 4 mins or 6 or even 12 isn't the real issue though. You can adjust the overall speed of your TL once compiled. Of greater note is logistics.

To answer your question:

1. Exposure time, under ten seconds, should be appropriate to correct exposure and aesthetic. Unless you are shooting at a very high speed, shutter drag will be apparent. Personally I think drag would look better in this case. It will enhance the sense of motion. I did something like that in the opening sequence here.

2. Same as above. This is a personal choice. The actually length of the exposure will not effect the overall length of the TL. Of greater consequence is the processing speed of your camera. Can it handle a 5 sec exposure every ten seconds without filling its buffer? My D100 can not, though my D700 can.

3. Personally I would. Even with shutter drag, a 10 sec gap "may" be too much for the eye. In other words, you run the risk of creating a choppy experience. I would do a quick test on your local highway at cruising speed and see how it turns out. When I TL clouds, I shoot every sec or two to minimize that gap. Certainly the prospective on clouds will be relatively slower than scenery on a highway.

Hope that answers some of your questions. Now here's a few other things to consider (from experience):

-Power Supply; how are you powering your camera throughout the trip. That continuos draw on battery from shutter and processor will kill them off quick. Make sure you're plugged into your A/C adapter.

-Mounting; your camera has to be firmly locked into the vehicle. The smallest bump (pulling into a gas station for example) can knock your camera off axis and then the whole last leg of the trip is for not. A bump of the camera, even the smallest one, is TL no-no. Also, you have to be able to move around your camera without bumping it. So make sure you have room to grab your munchies before your start your journey.

-At night; now that you have your mount squared away, what to do with your camera when you sleep. Nothing would be worse then to wake up to a broken window and stolen gear (again from experience). They are out there and they will steal your rig. Make sure your mount is secure enough that you can remove your rig in a way that allows you to precisely maintain your composition.

Good luck and happy hunting. I'd love to see your finished product.