I am interested in improving the air sealing and insulation in my house in climate zone 5.  The house has an unfinished attic with two dormers. The roof of the house slopes steeply and the attic roof rafters continue down from the attic to the ceiling of finished low height space on the 2nd floor. There is no blocking or air sealing between the attic roof rafters and the sloped 2nd floor ceiling. The framing for one of the dormers does not have a top plate and has no air sealing between the attic and bathroom walls below. There is mineral wool insulation in the attic floor that continues to the low height 2nd floor ceiling and dormer walls below. The house has no soffit vents, but has two vents in the gable attic walls and a ridge vent at the top of the attic. I think a lot of air and drafts migrate from the attic down the walls/ceilings in the winter time.  I am trying to avoid adding baffles and soffit vents due to the amount of work required to remove and patch the 2nd floor ceiling  

I have a few questions about how best way to handle the cold drafts that I think are coming from the attic:

1) will adding blocking and air sealing at the attic floor between the rafters create issues with the 2nd floor ceiling?  If I make sure this is well air sealed, will that make this a unvented cathedral ceiling? 

2) if I air seal around the blocking how do I know if there will be adequate air flow/ventilation at the underside of the  roof sheathing? The two gable vents are at opposite ends of the house (~35 ft apart) and the ridge vent covers the entire length of the house

3) do you recommend staying with a vented attic with an air sealed attic floor and new cellulose insulation? Or should I spend the money for closed cell insulation,  batt insulation and an ignition barrier in the attic ceiling? One consideration is that there is a heating and cooling air handler and ductwork in the attic.