To get near the insulation level you want, you would need to spray(have sprayed) foam insulation into the cavities. Furring out everything you would need would be impractical.
With the foam insulation, you could first install the baffles from the soffits to the ridge if you wanted. Open the ridge for a continuous vent and install the correct amount of soffit vents to balance the system.
It probably wouldn't hurt to install a power vent in a common area to get the super heated air out of the space.
I see no reason to have insulation at,"b" and there should be insulation at ,"c" where it's over heated areas.
Ron

You might try insulating the heating duct.

I think the highest rated foam boards are polyisocyanurate at R-7 per inch. You would need 2 x 8 framing to achieve close to R-49 all over the attic. You don't have that in the dormers. I guess you could install the boards in the rafter bays and just spray the dormers to get an even insulating envelope.
You don't mention the roof rafter size. You'd have to furr them out to get your stated R-49 result( if less then 2x8). You'd also need to build out the knee walls to get an increased R value there.
Unless you uniformly do the job throughout the entire space, you'll have excessive heat loss/gain.
I'd bring in a HVAC guy to evaluate the furnace and the ductwork. If the area can't be heated by the current setup, the cheapest solution might be those oil filled space heaters.
Ron

I had a gravity hot air system in my house from 1891. The volume and temperature of the air required to make it work was unbelievable. Modern systems using lower temperature air and small ducts are entirely dependent on fan power.

Warm air will rise up stairways but only if they are large and unobstructed and doors are left open.

I would have an HVAC specialist look at the system. Insulation can't keep a space warm if it doesn't have sufficient heat in the first place.

As far as insulation is concerned, I would suspect the roof/ceiling insulation.

I have a similar situation. i insulated A & B, but have since been told I should have insulated C, not B. However I want to use the floor area for storage. Do I need to rip out the ceiling insulation and put it on the floor? In my house G is also unknown, is blown in insulation my only option if I don't want to tear out the finished ceiling of the living area?

how big (approx) are those attic spaces?

Tiger, I'm struggling with that, too: D, G and H are pretty much unknowns, done during the dormer remodel sometime between 1940 and 1970. As Mightyanvil says, I suspect those areas are contributing the most to the hot/cold problem, be it through insufficient insulation or lack of proper venting.

This house used to have gravity heat - the floors still have the grate marks - goodness knows how that worked for the original owners in the '20's, since the entire house (save the later attic additions) was uninsulated when I bought it. I salute you Mightyanvil for making that beast liveable.

(On the other hand, I once lived on the 7th floor of an ancient Victorian apartment house in East Germany with one coal-burning stove for 6 rooms, and had to lug buckets of coal up all those stairs 2 or 3 times a day. Compared to that, a cold attic ain't so bad ... )

I guess my only option is to rip through the ceiling to take a look (if I can see anything at all besides rafters in my nose), but we just got done re-plastering and painting the entire finished attic space. Ah well.

I wonder the same thing about blow-in up top, Tiger. I installed blown-in cellulose in the walls of the entire house, but am hesitant to blow anything (foam, fiber or cellulose) up into the top roof spaces D and H lest I fill any kind of ventilation baffles up there. I don't see a ridge vent or eyebrow vents anywhere - what are the chances there's even any ventilation at all? Does it even matter for such small enclosed roof space? Is it roasting in the summer more from no ventilation or from inadequate insulation?

I too want to save my side attic spaces for conditioned storage, because it's about the only storage space in the whole bungalow.

Energy_rater, my attic runs the entire breadth and width of the house, 24' x 36', and is completely finished save for these two triangular side corners that are roughly 9'x8' each.

You are on the right track in determining where to locate your thermal barrier. if you didn't want to use these spaces you would insulate and seal the knee walls and
floor (sorry..a & c )and make that the thermal barrier.
Since you want to use the floor space then you have to
move the thermal barrier to the roofline.

I can see that you would need to insulate c..and think that this has to do with some of the comfort issues.

that you can not access c makes it more of an issue.

Even if you used spray foam insulation @ R-7 per inch
you would still need 7" to achieve R-49.
Do you have framing that will allow that?
The benefit to spray foam is that there will be no air movement, and less will perform better than the full amount of fiber glass batts..follow me?
So possibly you could get away with less than 7".

I would take up sections of the floor and see what you have to work with. You could cut out sections and replace
it with same size plywood. Then you would know how to proceed from there.

Have you thought about products for diy like tiger foam?
I've seen 2 part mixes for sale online.
for approx 150 sq ft one set should do you.

The benefit to foams is that they eliminate air movement.
Most insulation does not preform due to air movement through the insulation.

There are climate specific issues with vapor barriers for
cold climates that I am not well versed in, it would be for others with that background to help out with.

I'm curious about the fiberglass batts on the roofline.
We don't do that here..
And the 5" gap..can you see the underside of the roof decking? Is the insulation dirty on either side of this gap?

And one last thing..you could make an attic hatch to take
a peek at the attic space above the rooms.
Start with the cutting of the floor in those attic spaces
and graduate up!

Best of luck. You seem determined to get this right, and
there are some very knowledgeable people here.