In a large, complex building with multiple materials and fire pathways (a firefighter could probably produce a better term),

"fire pathways" works OK. There's no one definite term.

the patterns of how a fire spreads will be logically be more complex. Reconstructing those "pathways" and patterns becomes even more difficult when the large building then collapses.

Oh yes indeed. Fire can be sneaky enough without thousands of gallons of jet fuel going all over the place.