Just was wondering if I could play halbarad and arathorn in the same company(the grey company)?. And also more importantly could someone please tell me the point of denethors broken mind??. I just don't get it, sorry for my naiveté .

Halbarad and Arathorn are both upgrade heroes and can only go in their own particular formations. They are not Epic Heroes and so may not move around the way Aragorn, for example, does.

As for Denethor's ability, I think it is mainly for fluff reasons. I believe the character does something similar in the SBG and it may be more useful there. They did that a lot with WotR, simply made a rule or ability similar to what already existed in SBG with no thought to whether it would actually work or be a good idea. Denethor's ability does circumvent some of the drawbacks of being hit with Courage draining spells and the like though. It is arguable how he interacts with Spirit Grasp though, I can see arguments both ways.