What you show should work, provided your USB interface module doesn't interfere with the PAC SWI module. BTW, there are other steering wheel interface modules besides PAC's that can connect directly to a car that uses CANbus (for example, Metra's Axxess ASWC-1).
As an alternative, you might look into using a CAN-to-USB adapter, to interface your tablet directly to your car's CAN bus wires. You may have to do some work to decipher the car's protocol, but it could open up access to far more data than just the SWC commands (e.g., engine performance, fuel consumption, status of various systems, etc.). It might even let you send commands back to the car -- to control various settings and functions.
You'd want to take precautions, since that could potentially let a hacker do things like remotely unlock your car (via the tablet).