6. How can SOM-301U use external high accurate GNSS RTK from the host RSU platform ?

If the using case is that the host platform with RTK generates RTK data as BASE role, and V2Xcast distributes RTK data by RTCM messages to the nearby OBUs which play ROVER role, then RTCM message provided by V2Xcast SDK could support the case. There is a sample application for this in the off-the-shelf SW package.