Customizing Case Resolution Form

Dear House,

There is an Urgent need for me to Calculate the Time Difference Between The Date/Time the case is created (Created on)  and the Date/Time  the Case was resolved(Modified On). and Display in a Field = "Case Duration"

1. The request is made to Remove the Red Stroked fields from the case resolution form as seen in the screen shot below.
2. Auto-populate the (“Total Time”) field with the Date /Time Difference ( "Case Duration") 
3. “The Remarks” field should remain the only input to be entered by the user.

Which Customizations on this Form is strictly restricted as all the tools of the form

Kind help with some Fix or guide around this .

Thanks And Regards.