Main issue here was that in old usercount system 'user_real' did simply not allow that kind of thing to work. With new pait of 'USER_EXTRA' tags, it becomes possible to handle the case correctly, by merely refining checks about indirectly use objects whene removing them from a scene. Incidently, found another related bug, 'link group objects to scene' was not incrementing objects' usercount - bad, very very bad!