Is there a setting or a module, this would facilitate re-use of menus...
Menu 1 option A option B option C option D Menu 2 option 1 option 2 option 3 option 4 Full Main Menu option A (use menu 2 for sub menu options) option 1 option 2 option 3 option 4 option B option C option D
There are not settings I am aware. I have not done an exahaustive search for a module. However, you could write your own implementation with a custom module implementing hook_menu(). Simply put, you would create a new menu. Then, you would iterate through the other menu's in your implementation adding each element to the new menu.
Answering my own question, the closest existing solution appears to be Advanced Menu:
From the Advanced Menu module project page:
Sub-Menus with Menu Merge (7.x-1.3+)
Menus can now be merged into other menus. This allows users with the appropriate permission to add links to other menus which are then displayed as if there were a single menu.