I've now tested the build instructions by following them step-by-step to make a functioning device.

The good news is that there are no errors in the circuit itself. However, I had neglected one pretty important detail:

For the PCB to fit inside the 3D-printed inner case it's edges needs to be beveled. So there should be an instruction to sand down the PCB edges before soldering components to it. The instructions later says to place some components (power switch and C2 cap) right at some of those edges.

It's weird how you can miss obvious things like that when your mind is occupied with connecting the wires to the right spots and whatnot. Of course, once I started building it I realized I'd made a mistake immediately.

After finishing the build I also realized that there is some room for improvement of the component layout: 

For my build I used the optional dual pin header arrangement for the patchbay (build instruction step 10). After using the patchbay I think there should be a bit of space between the two pin header rows to reduce the risk of resistor legs touching by mistake. That means all other components has to move down one "row" on the PCB. I'm also going to place the ground wires under the battery holder.

So I'll hold off on updating the build instructions until I've incorporated those changes. For now I will include a link to this log in the instructions.