May I ask what kind of cable you used for the connection between the breakout board and the headphone jack?

We connected 4 wires from the RS232 breakout to a TRRS breakout as follows: RX<->TIP, TX<->RING1, GROUND<->RING2, VCC<->SLEEVE. Then we connected a male-to-male cable from the TRRS breakout to Nexus 9.

Thanks! Wasn't aware of the TRSS breakout board!

You welcome!

Did you try removing the front camera flex cable (internally) to enable debug accessory mode on the Pixel C?