In case somebody might have similar issue and settings:
I found rndis as solution for any useable inet access at least.
Docu here: http://docs.ubports.com/en/latest/userguide/advanceduse/reverse-tethering.html
I had to set link up and give address manually. But jippie, it works. 🙂
(WiFi stopped working. Was not able to get it working again.)
I got inet connection on the phone, but it seems to not use this connection for "fetching channels" - wheel's turning, no traffic seen via tcpdump on the rndis interface of the pc.

Still: Some image/packages to download would still be great. 😉