thanks, daverave, you are always there to answer! could you see it's from a scan by the horrid layout or why?

could you elaborate on these steps you mention?
I'm very much in need of a better workflow for fixing donut-holes etc in 3D-scans, but I could not figure out how to e.g. use the original uv on the mesh remeshed in zbrush and still have it align properly.
tutoriallinks also very welcome.

SOLUTION (for the original problem, UVs stretch other UVs)in case someone wonders:
with the faces to be separated selected I chose "Split UVs" (not Cut UV Edge). That did free them (put them in an own shell, if that's the right terminus). If not I delete these UVs and apply an automatic mapping (create UVs) and go again from there.