If the car has studs then the hub centre is not involved with the security of the wheels you would never fit a wheel that cannot be secured and this would be an issue with a rubber fitting, it sounds like a dust shield for the wheel bearing, not any sort of an expert, best nip along to a dealer and ask one of the fitters, clearly the car drives around without it but dirt ingress would turn to a nice grinding paste and the bearing life will reduce.
Mag wheels are expensive and can hide damage but lack of true would show up on a balance machine, if the wheels wobble at set speeds can be due to lack of natural damping in the steering, this is called shimmy, on commercial vehicles it manifests itself at slower road speeds but that would be expected, look at the tyre treads, the rubber blocks need to work this can be associated with a lack of a steering damper, too much freedom in the top kingpin bush or worn tyres,
