Hi,
1) FD-active node (MCU+transceiver) can simply do CAN FD communication.
FD-passive functionality is the ability to ignore CAN FD frames while waiting for a valid wake-up frame in Sleep/Standby mode. This feature of partial networking is useful on networks that support both CAN FD and standard CAN 2.0 communications. It allows normal CAN controllers that do not need to communicate CAN FD messages to remain in partial networking Sleep/Standby mode during CAN FD communication without generating bus errors.
2) This register is for partial networking configuration. In normal active mode TJA1145 enables reliable communication in the CAN FD fast phase at data rates up to 2 Mbit/s. No configuration is needed.
BR, Petr