So here comes the dumb questions
How big are the lvl beams, how heavy are they and do you have a plan for getting them in place?
How high is the foundation walls from footing to sill plate?
You are replacing the trusses with rafters and ceiling joists, full length rafter? Could be tricky to get in.
2x10 rafters is the standard today, increases the insulation value by 30%
How high is the seat cut of the trusses? The seat cut is the distance between the top outside of the outside wall and the bottom of the roof sheeting.
The trusses or ceiling joists hold the outside walls from pushing outward by the rafters, so when you open the hole for the staircase having a few ties across the hole until the subfloor is installed will stop any unwanted movement. The plywood subfloor glued and nailed or screwed will also stop that movement.
Build the knee walls out of 2x6, for insulation value.
Have you had an engineer look at this?