Thank you very much for your quick reply, Peter! I did indeed stumble on that diagram, but it was actually what caused my confusion to start with! I noticed in the *pins.h variant file for the Pro that USE_SOFTWARE_SERIAL was defined, which in turn disables most of HardwareSerial.cpp, so I was certainly confused. As well, I did not understand why the native USB on the chip would be used with Serial, and the other pins would be Serial1, but it sounds like maybe that was a misprint, from your description. And on further inspection, the top of that image does say "The Official BETA", so perhaps it was an older revison. But anyway, I'm rambling now. I will try PA0/PA1 with an 5V FTDI cable tomorrow, thank you very much again for clearing that up for me. Cheers!