3. How can OBU-301U use external high accurate GNSS RTK if I connected the OBU onto the host platform through Ethernet ?

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.