Question:  How to best manage summer chimney effects in a open design home space where heat rises to 2nd floor via a stairwell and becomes trapped in the room(s) above?

We live in a NW facing semi-detached end-unit (Philadelphia suburbs, Zone 4a) townhouse, developer-built, 2003.  Typical of contemporary design, there is a 2 car front-facing built-in garage.  To the left of the garage is the front door which opens to a 15′ cathedral entry way and stairs to a “loft” bonus room over the garage.  Next to the front door is a front-facing living-room space, 9′ ceilings and open all the way to the rear of the townhouse, 60′.    Designed for 55+ residents, the townhouse is mostly 1 floor living, the bonus room is the only 2nd floor living space.  There’s a full unfinished basement, poured concrete walls, floating concrete pad.
Last year I air-sealed above the bonus room ceiling with Intello Plus,  Visconn paint and Contega HF, then laid down about 12″ of Rockwool – in an effort to keep the Loft warm in winter.  It’s now June, and I’m wondering what to do on hot days:  Yesterday was the first day above 90°, This morning it’s 8AM, outside temp 70°, but the temp in the bonus room is 80°.
Sure, I could rev-up the A/C to cool the loft, but I’m wondering what passive air-flow measures I could take to remove/redirect the excess heat from the bonus room.

Here’s my list so far, prioritized is order of cost/effectiveness.

1) Open a window.  The loft has front facing NW dormer windows…. Simple and no cost, but only OK if inside/outside humidity levels aren’t too different.

2) Install a door at the top of the stairs at the entrance to the loft/bonus room.  While hot air will hang around the top of the stairs and the cathedral ceiling at the entrance, with shut door, less heat will seep into the loft, true?

3) Install a duct & fan.  In May and June, our evening outside temps are often cool, <65°, but our afternoons can get hot.  Would a bonus room vent with fan help – one that pulls warm air from the loft and pushes it down into the basement or exhausts through the roof?  I suspect this is a bad idea… but why?

4) Air-seal and insulate the RIM JOISTS in the basement, cutting down on the draft from the basement?

5) The BIG JOB.  To reduce the chimney effect, should I install insulation over concrete below-grade basement walls?  Cover with 2×4 gypsum drywall, move wiring as needed?  Yikes!  A huge cost and effort.  I should install air/vapor barrier on the basement floor too, since I have no idea if the builder installed a vapor barrier below the slab.

Let me know if you would recommend an alternative or reorder priorities:

Thanks for whatever bits of advice anyone can offer.

Dan Bliss