Using the two metal vents to move air out of the actual room space will do nothing to keep the roof dry and cool. If you had designed the roof to be somewhat higher, you would have had the needed space to put in the false roof vault. Unfortunately, this problem has no easy answer. Over time, an unvented roof will decay. I'd forgo the idea to fully insulate the roof rafters, with fiberglass insulation. You might give some thought to using 1" foam held to the bottom edge of the rafters, allowing the air above to be vented? Not the perfect answer but one that will help if done correctly. Paul