Francesca,
Your input on this topic is greatly appreciated.
We have recently upgraded to Moodle 2.8.6+ and have exactly the problem you describe: not being able to isolate teachers with their groups for activities and resources inside a course.
I confirm that your workaround with "non-editing teachers" is valid.
Isolating teachers within a course is not ideal but we do get asked to do this sometimes.
I wonder what the Moodle Roadmap is for this functionality.
Any answers?