Hello, @Scidem
Thanks for your post.
1. Would you mind share more information regarding to J166, in which document/chart it was mentioned?
2. When the cable shown is plugged to the board, the LLCECAN0/CAN14 could be directly used with the cable, there is mark on each pair of the wire,

3. For actually using it under BSP, you may reference the chapter12 "LLCE support" of BSP44 UM for S32G3, the detailed configurations, building prerequisite are provided.
4. The LLCE_CAN and FlexCAN are different IPs on S32G3, connections on RDB3 is shown below:

5. The error mentioned may be the LLCE_ERROR_COMMAND_NOTSUPPORTED, For the detailed error code description, I suggest referencing the Llce_Fw_ReturnType in Llce_InterfaceFwMgr.h directly
6. No, as mentioned, LLCE_CAN14 and FlexCAN0 are different. they could be used separately.
BR
Chenyin