I think that CAN-BUS Shield I linked has an Arduino library on Github, complete with examples you can reference, when you're ready for that step. You mention that most of your dev experience is in ...

It seems that access to any device on the CAN-bus gives them unobstructed control of the car’s systems. Any device can send commands to any other device.

The “N” in CAN stands for network, after all. The trick is to create a device ID for each desired CANSerial interface, which is done in his library using the usual Arduino setup step.