First analysis showed clearly, I destroyed a functioning 90€ keyboard.
In the beginning I had the hope that I could use the existing pcb traces but realized soon that I would need to handwire the complete board.
I found a good spot for the Arduino micro clone (Atmega 32u4) but had issues with the micro usb connector.
The pcb finally ended up simply as carrier plate for the cherry mx black switches. I disconnected all existing pcb connections to be able to handwire the board. In addition I used the metal wires within the switches to solder the rows - probably more an aesthetic decision.
By soldering the two spacebar switches in parallel only a single keypress is registered when the switches are not hit at the same time. It actually works quite well and the space bar sits stable in the keyboard. The only part that I will change in the future are the switch types. Two cherry blacks require 2x60cN activation force, which is simply too much. When hitting the space bar not centered only one switch gets activated but when hitting in the center it simply requires too much energy to push the switches down. Probably two cherry mx reds will work better.
Everything is connected and tested.
I finally placed several 5mm rubber spacers on the underside to level the board. The salvaged IDE cable (grey) that I used for wiring is quite thick.
On the top side I routed the usb cable between the switches to the backside of the board.
I built a simple oak case which is only 21,5mm tall. At the backspace bar (tallest point) the board is 28mm thick. Overall dimensions: 297mm x 108mm x 28mm
Almost done - flashing the firmware.