OK, if your termination letter states the 14th, then go to labour a few days before and cancel your work permit.
Then go to immigration and cancel your extension, it will show as being cancelled on the 14th, so no worries about overstay or new denied extension.
According to STM2 which you signed, you must inform immigration if your circumstances change,(whether on your last day or not).
If you don't cancel your extension, then further down the line when you come to extend your DTV, you could find yourself in for a 20k fine.