Looks like you are doing a good pre-plan. Suggestions:

- more water (it's cheap & the car can easily handle the weight)
- headlamp + batteries (for fixing that flat at night)
- signal mirror (spectacularly effective in desert air)
- cheap folding chair so if you are hiding from the sun under your tarp, you don't have to sit on the hot ground. Keep in mind that a sil-tarp won't block the sun nearly as well as a cheap blue tarp.
- fleece blanket for spending the (mild) night in the car
- cheap cooler packed with ice

The bulky things are available cheap at any Wal-Mart or Target.

Another thing to ponder: though you don't plan to strike out on foot, consider the possibility. I understand you have a PLB and don't plan to drive off-road. But if you are on a seldom traveled stretch of road, out of cell range, and break down 10 or 15 miles from a town or something, are you really going to pop that PLB and call for a life/death rescue? Or would you consider walking the miles back to a major junction? If so, you have to plan in a whole additional way.

Not saying you should plan to walk, just think about it. It will influence your gear choices, and the form factor of that gear.