Alright, alright, my Fairphone 2 is alive again! 
I thought I had done everything I could and read every guide, but this seems to have been untrue…
🇬🇧 🇩🇪 ✏ Installing the Fairphone 2 from scratch in case of failure / Komplette Neuinstallation des Fairphone 2 im Fehlerfall and https://support.fairphone.com/hc/en-us/articles/207914363-Manually-install-Fairphone-OS-for-the-Fairphone-2 were very helpful.
Not before I installed TWRP Recovery manually via CMD on my phone could I proceed any further, but this did the trick. After the installation the phone rebooted normally and came to life again, wooo-hooo!
Thanks for every answer and hint from everybody! 
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