Ok, the point 1 was a simple laptop OS-user permission problem so I would say my fault or my own foolishness (never tested when I was in fastboot if fastboot devices works just tried to install something instead and it did not show any permission problem). I used the opportunity to flash the latest TWRP
my-user@my-machine:~$ adb devices -l
List of devices attached
fbcfb651 device usb:1-1 product:FP2 model:FP2 device:FP2
my-user@my-machine:~$ fastboot devices
no permissions (verify udev rules); see [http://developer.android.com/tools/device.html] fastboot
root@my-machine:~# fastboot devices
fbcfb651 fastboot
root@my-machine:~# fastboot flash recovery twrp-3.1.1-0-fp2.img
target reported max download size of 536870912 bytes
sending 'recovery' (12204 KB)...
OKAY [ 0.386s]
writing 'recovery'...
OKAY [ 0.281s]
finished. total time: 0.668s
root@my-machine:~# fastboot reboot
rebooting...
finished. total time: 0.055s
So now I can give the Android6 upgrade another try. It feels that the only way is this very strange procedure with the downgrade in between, right?
I’m still confused a bit what would be the best way to proceed. The bug node describes a lot different workarounds: [Bug] ✏ Upgrade to FP Open 17.06.4 renders phone useless (freezes once booted and reboots itself)
One with Destroy the LUKS Header the other one without. Still confused what would be the best option. I think uninstall or at least disable xposed seems to be a good idea (but I did that before my first attempt as well)
Any last tips before I give it another try?