Update:

If I leave the UART powered, but cut the traces that connect it to the RX/TX lines on the bluetooth chip, then I get clean serial input/output between the Arduino and the Betz board. And digital I2S audio is working great. However this seems to have induced another intractible problem...

Now the Pin 51 reset line isn't working to power up the chip any more. It boots into slow blink mode and I have to push the reset button on the Betz board to wake it up.

I have tried:
- Disconnecting the DTR line that connects the UART to the RESET line by desoldering C4. Does not fix it.
- Trying different timings of setting Pin 51 High, including up to 1 entire second long. Does not fix it.
- Changing the voltage divider on the Pin 51 reset line to let it go up to 3v instead of 2.5v (Did this with a 10k+15k voltage divider though I'm not sure if those are correct for this). Does not fix it.

Blech.