"As I read it, if you have a handgun in your glove compartment, or concealed anywhere else in your vehicle (other than on your person), everyone in the vehicle is considered as carrying concealed"
I don't think that is an entirely accurate conclusion. An example of your comment not being correct would be the discovery of an "illegal substance" (outside of "Mary Jane") in ones vehicle. It may well belong to the passenger in the front seat or the hitchhiker in the back you picked up at mile marker 265. Should they be in denial it is theirs you most likely will be the only one taking the ride. In short, you da' Captain of the ship.....
Furthermore, in the criminal court of "beyond a reasonable doubt", the DA is really going to have a hard time proving who was legally or illegally carrying concealed. Then there is the final issue of federal transportation of a firearm laws i.e. loaded, cased, locked, etc.