You would set the roles for other users to prevent accidental deleting of indices.
With manipulating logs is not very common once indexed. You would need to secure you Graylog server. The person who is orchestrating the security policy/s for this environment, hopefully would have a LPIC-3 Security at least.
If you are worried about security within this DMZ. I would highly suggest making GPO’s for your environment to prevent users for doing such thing as sabotage or other accidents happing.
On a side note, with Graylog enterprise version you could sync you users roles (Teams) but you have to keep it under 5GB a day.