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I think i have answered my own question after reading your wiki. I have such ordered a couple of USB to UART programmers.

A Yes, and you appear to know how. (But you only need one programmer!)
B. No, it will tell you only the imbalance current, which should be (but is unlikely ever to be exactly) zero.

You will very likely get negative real power on phases 2 & 3 with the single-phase sketch because the current wave is displaced more than 90° from the voltage wave, so it looks like generation.

Thanks Rob.

I ordered 2 because if I break my first, i have a spare :-)

So I assume I just set up the Arduino IDE, import the firmware I have linked to above, then load it to the tx?

Yes, that's about it. But you really will need to calibrate it, particularly for the phase & timing errors. There are dedicated calibration instructions in the comments at the top of the sketch, but the general principle and technique is very much as in the calibration instructions in Building Blocks.
Be careful how you connect the programmer - look for the GND pins and align those.

Done, calibrated, looks good!