Making menu items active on desired page
Is there a way you could make the Context module a dependency for your module and hook into the Context module to do this?  Context already allows you to do this exact function.  You specify conditions and reactions when those conditions are met, per context.  In your case you would specify particular paths as conditions, and then your reaction would be to simply activate a particular menu item.  I would seriously consider checking it out rather than reinventing the wheel.