Hey guys,
Thanks for all your awesome work!
For everyone that is having issues installing stuff with fastboot (permission denied etc), especially also not beeing able to boot twrp (fastboot boot twrp.img):
Just download the full dump linked above, unzip it and flash everything back to stock. This will take a few minutes (prox. 10-15).
fastboot --set-active=a
fastboot flash product product.img
fastboot flash keymaster keymaster.img -S 522239K || exit 1
fastboot flash cmnlib64 cmnlib64.img -S 522239K || exit 1
fastboot flash cmnlib cmnlib.img -S 522239K || exit 1
fastboot flash lksecapp lksecapp.img -S 522239K || exit 1
fastboot flash mdtp mdtp.img -S 522239K || exit 1
fastboot flash vendor vendor.img -S 522239K || exit 1
fastboot flash system system.img -S 522239K || exit 1
fastboot flash boot boot.img -S 522239K || exit 1
fastboot flash vbmeta vbmeta.img -S 522239K || exit 1
fastboot flash dtbo dtbo.img -S 522239K || exit 1
fastboot flash dsp dsp.img -S 522239K || exit 1
fastboot flash devcfg devcfg.img -S 522239K || exit 1
fastboot flash tz tz.img -S 522239K || exit 1
fastboot flash rpm rpm.img -S 522239K || exit 1
fastboot flash sbl1 sbl1.img -S 522239K || exit 1
fastboot flash modem modem.img -S 522239K || exit 1
fastboot flash aboot aboot.img -S 522239K || exit 1
fastboot reboot bootloader
fastboot --set-active=b
fastboot flash product product.img
fastboot flash keymaster keymaster.img -S 522239K || exit 1
fastboot flash cmnlib64 cmnlib64.img -S 522239K || exit 1
fastboot flash cmnlib cmnlib.img -S 522239K || exit 1
fastboot flash lksecapp lksecapp.img -S 522239K || exit 1
fastboot flash mdtp mdtp.img -S 522239K || exit 1
fastboot flash vendor vendor.img -S 522239K || exit 1
fastboot flash system system.img -S 522239K || exit 1
fastboot flash boot boot.img -S 522239K || exit 1
fastboot flash vbmeta vbmeta.img -S 522239K || exit 1
fastboot flash dtbo dtbo.img -S 522239K || exit 1
fastboot flash dsp dsp.img -S 522239K || exit 1
fastboot flash devcfg devcfg.img -S 522239K || exit 1
fastboot flash tz tz.img -S 522239K || exit 1
fastboot flash rpm rpm.img -S 522239K || exit 1
fastboot flash sbl1 sbl1.img -S 522239K || exit 1
fastboot flash modem modem.img -S 522239K || exit 1
fastboot flash aboot aboot.img -S 522239K || exit 1
fastboot reboot bootloader
Before you can boot into TWRP you need to check the value slot-success for both partitions:
fastboot getvar | grep slot-success
If the value is not yes make sure to activate the slot, and fully boot into the OS before returing to fastboot.
fastboot --set-active=b
fastboot reboot
adb reboot-bootloader
Otherwise just boot into TWRP with fastboot boot <twrp.img>
I hope this did maybe help anyone, thanks again to the devs. I was pretty stocked to follow all the progress in the development and happy to see it working now 
Cheers