Bug tracking etiquette and convention always seems a little different from system to system and project to project. I haven't used the DC Mantis setup yet, but in general the assignee of the bug is the one who marks "resolved" and someone else tests and marks "closed". In a professional development situation someone else is formally verifying and/or regressing bugs, in which case it would be marked closed by whoever is doing the verification (not necessarily the original reporter). In this case the conventions may need adjusting unless the reporter can be counted on to verify the resolution and close it. I doubt anyone has time to be the official DC bug tester, so maybe closing bugs just won't happen normally.
In most bug trackers I've seen you can't usually update the original bug *text*, although you can sometimes change other stuff like version, priority, etc. This also depends on permissions. But in any case if you have mode to add the usual convention is to do so in a subsequent comment/note because then you maintain the history of the bug. This is precisely what the note feature is for.
Finally, in my experience it is the developer that will mark something as resolved and specify a resolution/reason. This gives them a function to notify others when it has been taken care of on their end as well as the ability to mark as resolved bugs they decide not to fix ("featured"), feature requests they decide not to implement, etc. Bug *closure* is the step taken after resolution, and indicates that the bug fix has been tested and verified.