after the "resolution issue" I managed now to solve the "printer issue" as well:
I compiled the new gutenprint version 5.2.9 and changed the output format from ´A4´ to ´letter´ again and it finally worked for the Cups testpage!
No weired system symbols anymore, real pics and characters! wuhu!
the other open issues will be put under my second threat "hurray" - but ### happy, the printing now works!