(regarding the denim sound insulation and fire)

Maybe, but there's a pretty good chance it's treated with flame retardants before use, and cotton fabric generally doesn't really burn that well - or at least not fast.

There's a reason people working with hot glass wear long cotton shirts/pants for it.

I had a chemistry professor mention to the class she always travels with natural fabrics, cotton and the like, whenever she's onboard a flight. She mentioned something along the lines of it increases your chances of survival if ever there's a fire because synthetics melt and stick to your skin and burn like crazy.

I took some glassblowing classes and the first day safety lecture included a bit about this. Eye protection, closed toe shoes, and cotton clothes, precisely for this reason.

Especially when we were first learning, it was easier than you might think to spill or splash 2000F liquid glass on the ground when gathering it out of the crucible, and that'd turn synthetic clothes into napalm.

That's an ... interesting failure of statistical thinking. Fire in the cabin is by far less risky than many other potential harms he encounters en route to the airport.

Assuming your clothes fit properly, your choice of clothing will have little effect on your chances of surviving your journey to the airport. However, clothing that might help you survive a plane crash will likely have the same benefit in a car crash.

I used the term fire loosely, it was basically a plane crash and subsequent fire, an inferno in other words not a small camp fire around the center aisle seat.