Hi,
welcome on the forum,
1 - You could link ogre in theory, but Maratis itself is a 3d engine so it won't really be of any benefit (better to use Ogre alone at this point).
2 - The wiki is still in progress, there is some documentation for the script, but not much about the engine.
The best is to parse the svn code. Basically there is the engine : MCore + MEngine ; a gui lib : MGui ; and Maratis (MaratisEditor + MaratisPlayer). MCore is the core including all the basic libs (like a render engine), MEngine is a full game engine (objects, scenes, mesh system etc).
You can use MCore alone or MCore+MEngine alone, or from the Maratis editor.
Parse the docs and examples here : http://www.maratis3d.org/?page_id=53
3 - It's pretty capable. After you have to be aware that a game like assassins creed is 300 peoples and more than 10 millions euros of budget. The limitation is more this one than Maratis capabilities. You can code additional behavior in c++ to extend Maratis, to create AI for example, every gameplay is possible and there is a visual editor.
4 - You can add external lib and extend the engine by writing a game plugin in c++ (add custom behavior, special game class etc)
Look at tutorial "How to create a game plugin"
5 - I spend more than 8 years working on it before publishing it in open-source, it is now improved by contributors, donators, a lot of nice people : )
6 - If your game is made using the editor, there is a button to publish the game. You can also use MaratisPlayer by hand. Or if you want to control all the process you can publish a game using your own modified application that use Maratis SDK (look at "ManualUse" example on svn).