I have now assembled and programmed the PCB:
There are only two problems with it: the diode next to the "shift" key is shorted (because I moved it and forgot to move the trace), and the cathodes of LEDs are not connected to GND (because I forgot). Nothing a few bodges couldn't fix, though:
I have the basic firmware copied from the #Flounder Keyboard, and it works. Now I only need to implement the mod key magic, as described in the previous log, and it should be ready to be used. Oh, and of course I'm still waiting for the switches, but I'm confident they will fit because I have one left over from previous projects.
Finally, I decided to not try to carve up the switches to make the USB socket fit, and instead opted for a fixed cable.