Help me understand the advantage of insulating the attic roof?
First, since you have ducts, and an attic-floor-to-rafters room(!) in your attic, it's practically impossible to meet the conditions needed for a vented attic to work in your case.

Since I see you are in the eastern 3/4ths of NC, where ground snow loads can be guaranteed to be below 50lbs/ft^2 and in fact don't exceed half of that as per IRC Figure R301.2(5), a compact over-roof is how I'd solve your problem, instead.

Why a compact roof?

There are two conditions needed to make a vented roof work.  The first is that there must be a barrier plane at the ceiling capable of keeping heat and hot (or moist) air from entering the attic and warming the roof deck (or condensing out on the bottom of the cold roof deck -- hygric buoyancy is a thing).  This barrier must be as good thermally as the wall is directly over the exterior wall's top plate (i.e. R-30 over the top plate for a R-30 wall) and it must be airtight to the point where heated air can't get into the attic and spoil all your ventilation work.  How airtight?  Well, I'd be using air barrier tape to seal the ceiling drywall if I were building a vented attic.

However, with ducts in the attic, and a conditioned room in the attic as well that extends to the rafters at some points, the challenge of building a thermal and air barrier between the conditioned space and the unconditioned attic/roof deck just went from merely "hard" to "ARE YOU KIDDING ME?"  Ducts leak, despite all the work you do to mastic every last joint (that only gets your leakage down to a few percent).  Worse yet, the existing drywall isn't air barrier sealed I bet -- even with sealed and insulated ducts, a drywall joint that has not been taped specially to serve as an air barrier will still leak air.  Never mind that door to the unconditioned attic (err...storage space)!  

Second, the underside of the roof deck needs to be vented to the outside air.  This is how the roof deck is kept at the outside temperature, no matter how much snow is piled atop it.  In practice, this usually translates to venting the whole attic, although it is possible to use an a rigid board insulation (foamboard or rockwool) sandwiched between the rafters and a layer of drywall attached with long screws into the rafters and air barrier taped, effectively creating a vent space while conditioning the attic.

Furthermore, vented attics need copious quantities of attic ventilation as ventilation's a lousy way to dispose of heat -- if I were building a vented attic, I would strongly consider using deep/wide enough trusses for the roof that the requisite top plate insulation does not come up to the bottom of the rafters, along with the "insulation wind baffle" from the illustration below (taken from BSI-046):



Given that the eastern 3/4ths of NC doesn't get enough snow loading for the R-value of snow to be a factor in ice damming, the commercial-style compact roof makes more sense in this application as it moves the attic into the conditioned space instead of trying to build a conditioned, air-sealed room within a vented-to-outside-air attic.

How do I actually do this?

Implementing this isn't hard, provided the details are minded.  I would strip the existing roof down to the deck (which can stay provided it is in good condition), then apply an air barrier layer i.e. a fully adhered product or a liquid applied product, as sheet products get rips and holes in them that make them poor air barriers.  Atop that, I would put in at least two layers of rigid insulation board (rockwool or foam) with a total R-value of R-50 or better and the joints staggered both horizontally and vertically to control airflow (otherwise, weird, complicated airflow networks make life hard).  Finally, I would attach an overdeck or nail-base to the rafters with long screws as the insulation will resist the bending moments of the screws, then put down whatever actual roofing system I like for the application as everything's taken care of below it.  See the illustration below, taken from the first link above (i.e. BSI-046), for a graphical view.



And oh, last but not least, once I fixed the roof, I'd remove the insulation from the attic floor -- it'd be superfluous once I installed the compact roof, as the attic is then properly part of the conditioned space.