I build a new, 2cm tall, gaming keyboard using cheap low profile switches and 3d printing the rest. All files and mores pictures can be found here
You can use any pins from D0 to D21. You can find out where each pin is by searching for "nicenano pinout". You will have to change the firmware later to fit the chosen pins.
If you already have a Firmware you can find the pins (at least in QMK) in the info.json or config.h file. I have never used ZMK but the used pins have to be in one of the files.
You can just make up some e-mail as, without internet, you couldn't verify it. Also one of the rare cases where VPN directly improve your privacy.