Quick updates, because better to be honest out loud, it seems:
- 1.3 arrived and Iāve built it up.
- all the MIDI is good, USB and jack; all the voltages are good
- I need to test I2C again - there have been some slight changes to I2C implementation in 1.3 so Iām not just going to assume itās good. Thatās going to wait around a week or so, because Iām on vacation this week.
Here, have some pictures to explain why 1.3 is an improvement:

That nice big SOIC chip to the right of the Teensy is the multiplexer. That meant I had to reroute the entire damn board, and move all the little ICs along. But!

thatās the payoff: just two rows of Teensy pins on the back (this board is upside down compared to the previous picture). You can see all the multiplexer pins being broken out at image right.
I am pleased with this. Weāre actually nearly there!