If you slave a device it doesnt mean that the original user loses any privilages on that. It just means that you must have admin rights on that system or device, and that device loses the ability to communicate with independent devices wirelessly for the duration of the slaving, it must use the master node to pass all external communication through. I wouldn't think that if you slave a commlink it looses the connection to other parts of the original user's PAN. Furthermore, as the original user might have retained the admin rights most non-corporate users have on their devices, i would say she has the ability to un-slave a device any time by using the change linked device mode action.
If you think IRL examples, there are system setups that use clusters within the company network, individual computers within the cluster, etc. It's like a row of slaved systems.
On the other hand, i agree on GridLink. It is set up so that in some situations it is able to override a normal user and the car's commlink is not able to shut it out without shutting down or using a GridLink override. However, i think a hacker in the system might be able to terminate it's effect through rewriting procedures, defeating the agent in cybercombat, etc. Though he will be sure to activate some kind of alert if he is not cautious enough. (Spoofing a command is a good idea, though you have to have a valid access ID for that.)
And yes, the Blessed Trinity of spoof chip, morphing licence plate, and gridlink override is a must for runners in any car that has wireless capability, and which they want to use outside legally allowed limits in an environment that has gridlink. But we can't know everything, anyone might've overlooked this mod when buying equipment.