Here's a typical "Cape Cod style house" where the attic space is used as a second floor on the house, but the floor space of the attic is less than that of the main floor because of the construction of knee walls in the attic. You also have a ceiling in that attic space, giving you dead space above the ceiling as well.
I really don't see a lot of benefit in insulating the roof rafters behind the knee walls. The only potential benefit there may be in doing that is that it would cause any ice daming to form lower down on the roof, and closer to the eves. That would minimize the potential for water to get into your house if ice dams were to form.
But, if ice daming hasn't been a problem until now, (and you should have Ice and Water Shield that would prevent water penetration into your house) then there wouldn't be any benefit to be had there either.
If the dead space behind the knee walls is open to any soffit vents you have under your eves, (and you say you have three roof vents in the dead space above the ceiling), then it's important that you provide ventilation of the roof to keep it dry and prevent any wood rot.
I don't know what those styrofoam channels are called, but the idea is that you use the unavoidable heat loss through the roof insulation to drive a convective air current in those channels. That convective air current evaporates any condensation that might form on the underside of the roof sheathing and thereby keeps the roof dry. Dry wood is happy wood.
But, for there to be a convective air current in those channels, you have to have a way for cold air to get into those channels at the bottom (typically through soffit vents) and warmer air to get out at the top (typically through roof, gable or ridge vents).
So, if the kneewall dead space is open to the outside somehow, I'd install those styrofoam channels between your rafters and just insulate your knee walls as before. Then cover with 6 mil vapour barrier and drywall.