well, thanks for answering … i finally solved my problem as well in the end
it could somehow be made clearer (probably by some bold emphasis in the plugin doc or so) that you can not combine python-packages: with any deb based python packages added to stage-packages: …
creating an extra part that pulled in the stage-packages separately (i only needed a single pip based package that simply isnt available in the archive) and mangling PYTHONPATH in an environment stanza finally made it work … the snapcraft.yaml seems to feel more bloated and complex with each of these changes though …
and i also wonder if we could not detect that there is a stage package with python-* in its name and warn the user that things will break in her face …