4. How to use the Data from in-vehicle-network (ex. CAN) if I connected the OBU to the Vehicle PC through Ethernet ?

Data from in-vehicle-network required for facility and application layers should be collected from the main computer system. Then, to fill them into messages (e.g. BSM) and broadcast in the air through V2Xcast SDK. Hence, to collect data from in-vehicle-network will rely on current CAN interface and SDK of your main system.