I think this approach here for the whole interface would be the best.
Similar to Visual Studio (and other IDEs), you would have a few panels that you can move around and toggle visible and invisible (list of scripts, list of maps, resource manager, sound manager, properties panel etc.). One panel would be special as it would be the main panel and be able to have tabs, similar like the one for files in VS. Now the thing is that you can open anything here. You can open a database category (e.g. actors) or you can open a script to edit or you can open a map. We just have to make sure the same thing cannot be opened twice.
From reading your posts I didn't get the idea that it would be like this (though it would be similar or maybe I just misread your posts) so I thought I'd write out this whole idea. I've had this idea since I had the idea for StoRM. So the concept is that everything is either a panel that you can dock anywhere you want and toggle visible or something opened in a tab in the main panel to work with.
There's just one small special thing that I would like to explain better and that is the properties panel. I thought of a panel similar to VS's properties panel. The idea is that you can change the properties of the currently open tab or select object in other panels. e.g. If you select a map or have a map open, in the properties panel you change the map properties easily. If you have selected a script or have a script panel open, it would show the properties such as author, version, whether it's active and so on.
Again, I think this approach might be one of the hardest to code, but it would give the editor a feeling of an actual IDE.
If you look realistically at it, ARCed will actually be an IDE, it will just not have a shitload of features like e.g. VS or CodeLite or Eclipse does.