Sloped roof air barrier/strapping/mechanical cavity – GreenBuildingAdvisor

I am building shed roof design in zone 6 maine with trusses, double studd design (2×4) blown in cellulose, two story house.

-There will be a standing seam metal roof

-high temp synthetic roofing underlayment

-5/8″ APA sheathing, engineered roof truss system

-create a 1.5″+ air gap at top for venting with Accu-vent product

-22″ deep blown in cellulose for R74

-Siga Swiss Majrex 200 at truss bottoms – taped as ceiling air barrier and installed prior to erecting second floor interior partitions

-2 5/8″ (half ripped 2×6) strapping installed on edge at 16″OC installed perpendicular to trusses supporting Majrex200, strapping creates mechanical chase possible Rzr style lighting fixtures

-1/2″ GWB ceilings throughout.

Question:

1.   Does this system look good overall?  Suggestions

2.   Air barrier:   Thoughts on Majrex 200 vs OSB as air barrier?

3.   Air Barrier/Insulation Order:  Is builder constructing all walls/partitions/air barrier and then insulators cutting/drilling holes in Siga or OSB in ceiling in order to blow in cellulose?   Or is builder holding off completing partitions for entire second floor including air barrier for ceiling, then insulators do entire house, then builder returns to complete ceiling air barrier and construct all partitions?

4.  With sloped ceilings and an air barrier, in order to run wires for lighting between air barrier and drywall ceiling, what are people using for strapping to provide the needed cavity?   Also the depth of the wire cavity specifying?  Was thinking of slim LED recessed but concerned of lots of holes in drywall and integrity of the air barrier so now leaning toward surface mount lights with low profile electrical boxes as needed…try to light with more sconces.  I prefer to gain as much space so I can be just adding 8″ of space or something for a cavity.