1 - Yes
2 - Yes but that doesn't mean the OS will use it
3 - No, but this may vary
4 - It working in another device is irrelevant. The OEM and the carrier need to collaborate and deploy relevant carrier settings for each respective device. It's not really part of AOSP, there are configs in AOSP, but that isn't complete.
GrapheneOS just extracts the carrier config protobuf files which are used by the proprietary CarrierSettings app on the stock OS and converts them to the XML based carrierconfig format used by AOSP.