Era is welcome to use her Carbuncle to assist the investigation. The fire would likely make it more difficult to track individual smells - the overwhelming scent of burnt material would mostly mask any smell of the arsonist (and they wouldn't have left any clothing behind to use as reference, for example). That said, the arsonist themselves would probably smell like the fire, so that is a potential option to pursue. The trail wouldn't lead directly to them, but would lead to another clue (or clues) we can offer in the second infodrop!

The accelerant would be a little easier to sniff out. It's not a terribly uncommon substance, however. The scent of it would most likely be useful to identify if a suspect has some of the accelerant with them/where they're staying/if they got it on any of their possessions/etc.