Hmm, time to start looking at the SoftwareSerial library on Arduino... There's a good chance I can temporarily use D2 as a dedicated 9600 BAUD serial input (directly connecting pins RX0 to D2 so both get the same signal) and tell from what's received that I'm in
ICC initial boot mode vs booted & established 115200 BAUD mode.
There's a light at the end of this tunnel... Or a train heading my way.