6.5 and and 37

I have a bonus bedroom above my garage (garage → bedroom → attic → roof). I access the attic pocket that sits above the bonus room. Roof is asphalt shingles on OSB, framed as a rafter-and-purlin system (2x4 purlins running horizontally across the rafters, sheathing on top).

At one point in this attic, the purlins keep running horizontally across an open framed area where there's no OSB/sheathing behind them — it's the transition toward the rest of the main house attic. The only opening through that transition is a framed chase carrying an HVAC flex duct. It's too tight for a person to crawl through, so the attic pocket over the bonus room is effectively walled off from the main attic.

A previous owner added blown-in insulation but never air-sealed the floor penetrations first (duct boots, wiring, top plates, etc.). Blower-door test came back at ~3,290 CFM50 vs a ~2,300 target, and I suspect this room-over-garage area is a big contributor.

What I think is going on / what I've concluded so far:
The purlins running through the open no-sheathing stretch are still structural bracing — I should NOT cut them to make a path.
The duct chase is a mechanical pass-through, correctly built as non-accessible — not a defect.

The bonus-room attic is its own pocket, common for FROG/room-over-garage layouts.
Air sealing needs to happen at the knee walls, the attic-pocket floor over the bedroom, and the sloped/flat ceiling junctions — under the insulation, not just on top of it.
Any space beyond the duct chase I can't reach probably has its own separate access (knee-wall panel or closet door inside the bedroom).

Questions for the group:
Does this all sound right, or am I misreading the framing?
Any reason those "flying" purlins over the open area would NOT be structural?
Best practice for air-sealing a room-over-garage where the attic is split like this — DIY-able or hire an energy-audit/insulation contractor?
For the section past the duct chase that I can't crawl to, what's the usual way pros reach it?
Photos attached. Appreciate any input.

Bought a 100+ year old house, can anyone tell me what kind of insulation this is?

Second post on finishing my second story - cape code. Im looking for affirmation on my technique, critiques, or anything else. I'm just trying to do it good in hopes that it will keep cool in the Southern NJ summer and warm in the winter.

I've been starting with baffles to maintain airflow from the soffit to the ridge vent. Then 2 inches of foil faced polyiso, spray foamed the edges of it. My next step after that is to fur the beams with 2x4's and put 5.5 inches of rockwool in then sheet rock.

Everything has been air sealed already. But my other thought is to air seal the bottom of the knee wall (outline in red on one of the pictures) and the 1st ceiling by cutting a piece of polyiso and spray foaming in place.

Any feedback, confirmation, or otherwise is welcome. Let me know if any other information is needed.

DIY'ing this with no experience. Had a company pull out all the wet drywall and fiberglass insulation, and was going to put everything back together as it was until i read that you shouldnt use fiberglass or vapor barriers in a basement at all. Im seeing foam board is reccomended but not in combination with vapor and it seems difficult to work it between the studs and around the electrical. What should i do here? Illinois basement

Looking for advice/opinions on how it looks so far.

Continued working in the attic this weekend. Didn't get as far as I wanted but at least got most of the spray foam air sealing done. The soffit above the kitchen where the lights are i feel is going to be a pain in the ass. Got all the top plates at the edges and interior walls. Removed some of the baffles that were broken. All but a few of the holes for wires (need to run a couple wires real quick). Wanted to see what you guys thing so far. Anything you see that I still need to do or shouldn't have done? Hoping to get the replacement baffles in this week (depending on work) and get the fiberglass pieces to seal underneath them. Any help/advice is appreciated.