The first Core module is assembled with some spare parts lying around and is now tested, all gates I/O, all data I/O, encoder switch and rotation, all four potentiometers, the TFT-screen and power pins are working!
I'm going to skip the LEDs because of space issues. I have to use straight 3.5mm cables instead of angled for the same reason.
My decoupling cap (01uF) that I placed between signal and ground on the potentiometers did absolutely nothing to smooth the incoming signal so I'll have to implement a software solution.
The next step is to wait for headers and 3.5mm jacks to arrive from China so that I can assemble and test the Xtend modules.