the thought of unconfined napalm suspended above could make for at least a few restless night?

You want to fill the area above that wall, just between the floor joists? I'm not sure the faced fiberglass is worth the trouble. Are you trying to mitigate noise? What's that wall made out of, and how thick is it?

The welding blanket is a great idea. But, fiberglass, can you cut it without shedding fibers everywhere?

Avoid ANYTHING that will burn. You'll violate your new code compliance. You can get fiberglass insulation panels with a foil backing. These can be cut to size with a utility knife. Some good tape should hold it in place.

Do a search for "basement fire barrier" and you'll find other ideas.

Cheapest way i can think of: buy a celling tile case and a tacker, cut the celing tile un a T shape and tack them on both side between each of the truss. No need to put anything between them. That'll cost around 100$.

Forget the drywall celling for a while you should at least replace all the stuff that's gonna be stuck in there for peace of mind. Then you can think about it.

You should get that celling tile for the whole celling asap this will help with the soundproofing of the first level. Consider extra soundproofing after this.

https://www.homedepot.com/p/Owens-Corning-R13-Thermafiber-Fire-and-Sound-Guard-Plus-Mineral-Wool-Insulation-Batt-15-in-x-47-in-1310109/319880476

This is the kind of thing I would suggest, it's designed to be soundproofing, fire retardant home insulation. This is literally the exact thing you would put in-between your joists before drywall if you were trying to insulate an interior wall for sound.

There's a product called green fiber that is a blown-in cellulose insulation that's been treated with fire retardant you could use if the walls have already been drywalled, I've used that in all of the interior walls of my home when I was remodeling to try and deaden sound and have added privacy throughout the home. My basement, which I turned into a mother-in-law apartment, is so private I can't hear my basement neighbor when they have their video games blasting through a surround sound stereo set with a subwoofer. I can feel the floor shake but, unless I am standing right next to a vent, I can't hear anything.