Restore Android 6 on Moto G (2nd Gen) without adb?
The solution was to re-open the bootloader. It is possible to unlock it again by using the code provided by Motorola and the same command I used for unlocking the bootloader in the first place. I did not know it was possible to do that until a friendly person pointed it out to me. After that, I was able to (almost) effortlessly flash a stock ROM, and then (just for trying how it works) CyanogenMod 13.

Thanks for your assistance on this!