Currently if a user is a member of a roles assigned "Edit" capabilities they can add events and edit their own, but not edit events added by others unless they are in an adminitrator role. If they are also in the group assigned as moderators they can edit events created by others. This works, but it's somewhat limiting and is not intuitive if you are not using moderation.

It's limiting mainly because it's one group of moderators. Also, it would be great to assign a specific moderator to each event without giving them access to all of them. Some of this could be corrected if an adminitrator or moderator could "reassign" an event. Say I'm in an edit role and I create an event for a collegue. That collegue cannot edit that event unless they're in the moderator role. If I could both see the event "owner" and change it that would help.

The other limitation is mainly that of clarity. If you're not using moderation it's not intuitive that assigning a moderator role gives you that capability.

In any case your permissions grid idea sounds like a good one. If you combined that with an assignment option like that in the Documents model I'd think you'd be golden.