Permission System Problem

Hi Guys I am having a problem now with permission system but first consider this Navigation that I want to discuss about

Navigation
---------University
--------------Welcome
--------------Vice Chancellor’s Message
--------------Brief History
--------------Blah blah blah
---------Academics
--------------Faculty of Engg.
--------------------Dean’s Message
--------------------Departments
--------------------Chemical Engg.
----------------------------Undergraduate Prog
----------------------------Postgraduate Prog
----------------------------Course Outline
----------------------------Blah blah blah

What I wanted is that a particular user may edit the content for a particular section but must not entirely recreate the menus, In other words the menu should be fixed, and precisely what I want. But the content can be altered by different Users having same Role
For example I have created a Role ‘Chairman Chemical’ which is responsible for editing the content of entire Chemical Engg. Department only.
Now I want all the users that have this role assigned be able edit the content of particular section.
What I did was I created This New Role Chairman Chemical; and assigned him the permission to edit only his own Page content. But now there’s a problem with that only a particular user could be an author of the content so only he/she can edit it, what I want is all of the users that have Chairman Role assigned be able to edit. If I allow the permission of edit all content to the Chairman then He can edit all of the content which is not what I want.
In Simple words I want to apply restrictions on a Role not on an Individual User because users will be changed as time passes on. Anybody like to help me out ? I would be thankful
oh and this was continued from this post