Progress shot of the schematic and PCB design.
The PCB toner transfer was mostly successful. The result is salvageable by passing over broken and hair-thin traces with a sharpie. Also painted over holes in the ground plane which is definitely not required, but I thought they might be confusing while drilling later (and ugly).
After etching, the PCB looks usable. Sharpie fixups definitely worked; might need some solder touchups here and there to fix broken traces.
Running a few manual tests with DIP-switches and LEDs, after completing the assembly of the adder board.