I have the Black Vue DR970X-1CH, had a local shop install on the passenger side of the mirror. I don't need the parking mode and it was a 30 minute job.
Yes you can install the shelf without the side panels. They are sold separately on the website and you can also see the installation instructions posted there.
I still have the factory roof rails and while no longer on my GX, the stick up almost 3 inches from the roof. Add the height of the factory rack and you're probably close to 5 inches above the roof.
As for the asterisk, the Sherpa rack is about 2.5 inches above the center of the roof but the...
I don't have measurements but yes it is. the factory roof rails come off and the rack is bolted to the roof. I'm guessing but it is several inches shorter than the factory roof rails plus the factory roof rack.