after a couple of days I come to the conclusion, its not possible to establish another Wifi Device on a raspberry pi 3. The first one (onboard) is the only one which works, it is continues working even its bcm-module is on modules' blacklist and should not be loaded as module (but it is). The second device is there, in meaning its recognized and its module is active, but the wifisystem is of the opinion, another wifi should not raise.
It seems, no one is trying what I tried: No information at all can be found with help by google. Even information about how to disable the built-in and use an USBDevice instead is hard to find. And the ones I have found are not working.
Should I try another firmware image. Is raspbian broken? (the "no one can deactivate the screensaver"-problem points in that same direction)
Best Regards, Andre