2003 home, new over-range microwave questions
The 4-outlet is on a 20A circuit, the new microwave uses ~14A, which gives you about 6A for the bonus room.  The microwave will only be pulling the heavy load (MW) only occasionally. I would use the 4-outlet the way you described, with the other outlets taped over.  The original exhaust-furnace circuit at 15A does not have the capacity to carry the MW load.