I'm not sure why you can't select just the centre content area to be editable.
Once you have saved your document as a Template (Dreamweaver Menu > File > 'Save as Template') then you should be able to place your cursor in the middle of your content area div (inside the dotted lines of the div) and select <div#content> from the elements info bar just above the properties panel/inspector to select the div.
Then from the Dreamweaver Menu, select > Insert > Template Objects & select 'Create Editable Region'.
This will then be the only area that will be editable unless you add new editable regions to the template.
Click to save the template, and when if it asks you if it should update the links, click 'yes'.