Anywhere that there is a sloped ceiling a few feet off of the exterior wall, a beam is not required. Ideal Homes nails the joist to the rafter. Ideal Homes also nails a block of wood under the joist and nail it into the rafter. That is what is typical for the engineering in Oklahoma.
The 90lb felt is used in the valleys only. One layer.
The 2x6 wall question…. There is a foundation wall there, but more importantly Ideal Homes installs a 2x6 wall at the back of the pantry to have room for the water heater water lines with extra wall insulation. We also have a 2x6 wall in the utility room to fit all the drains, water lines, and dryer vent for the washer and dryer.
Attached is a new set of plans that will hopefully helps clarify any confusion.