As far as I know: forget this thing about the lowered metabolism. Your body needs the energy it needs for basic functionality.
You may feel less active, lowering the energy used above the basics, but still your heart, lungs, brain, temperature management and all the other stuff need roughly the same energy. If your body does not get it from food then it will use up the fat.
But eating this low level of calories you must make sure that you consume all needed vitamins, minerals and enough protein.
And being less active may end up in a decline of muscle mass. In the end that may lead to lower basal metabolsk hastighet, but not your metabolism shutting down.