For that I would probably have to “backport” the national roaming configs from newer Android versions to my build - so android_packages_apps_CarrierConfig/assets/carrier_config_23203.xml at lineage-16.0 · LineageOS/android_packages_apps_CarrierConfig · GitHub becomes android_frameworks_base/core/res/res/values-mcc232-mnc03/config.xml at cm-11.0 · LineageOS/android_frameworks_base · GitHub for example.
Can’t help you with the other question though (but thinking about it, I believe that the proximity sensor on my unit is also broken)