I've never understood how this can be limited in practice: surely as far as the carrier is concerned, all traffic from the mobile device is the same (unless there are identifiers on the traffic coming from hotspotted devices via the mobile device). Here in Australia we've never had any form of hotspot detection/segmentation - if you have a data plan, all data features work (across all carriers). I do recall lots of online chatter from the US though, especially years back when mobile data was more of a precious resource.

How does the carrier know that the traffic is being proxied for another device, and not e.g. requested from the phone's web browser or another app?

Does the phone add a proxy header? Can it be configured to not add the header?

Plus it seems unlikely that the telematics module is even really related to the display screen stuff, let alone being configured to use alternate network connections to transmit data.