Your question about setting just one Glossary activity to be editable got me thinking: Moodle locally assigned roles feature, which allows you to change the permissions on a given activity (or anything else in Moodle).
In my Glossary activity (while logged in as an instructor), I clicked on the "Locally assigned roles" link in the Settings block. Then click on "T.A. (non-editing Teacher)" and assign the students to that role. The result of the assignment is:
1. Students can now edit entries that they did not create (as long as the Glossary creator has allowed editing to be available in the Glossary set-up).
2. Students cannot change any of the settings of Glossary activity itself.
3. Students will not be listed as the "author" of the entry they edit. You will need to figure out some other way for students to be recognized for contributing to specific entries (if that matters to you). Maybe they can leave some sort of comment in the comments area for the entry...
Note that the new permissions are not global across the whole course site. The new permissions are limited to that specific Glossary activity and not other Glossary activities on the site.