Thanks brad for the Manjaro Architect-tipp. But like Manjaro itself it did not even boot. I think the problem could be related to grub going into graphical-mode and maybe i could solve it by altering grub-config in the iso….
But like I said: My last option is to sell the tablet so for privacy-reasons it is important for me to “dd” the emmc out before selling, so as of now I used the current arch-install-iso and was able to get wireless working using iw/wpa_supplicant (kernel 4.12) so currently I’m about to install Arch to the emmc but had to pause for an video-evening with my wife 😉
When Arch is up and running, I’ll be trying out the other parts of my problem-list, maybe waiting for 4.13 getting into stable-repos or installing it from testing if there are still problems but for now I’m happy that I could use the wifi-chip.