That's an interesting question. Reds can't cross a threshold, but as demonstrated, a well-tossed molotov cocktail does the job for them.
I think the answer has to do with how the fire is attempted, and the strength of the threshold. If a summer Fae tried sending fireballs against Murph's house, I'm sure that the magic would just splash harmlessly against her threshold. Lily's ball o' sunshine might get through in a very weakened state. Of course, a burning branch tossed against the house wouldn't be hindered.
But then, there's the question of what one can do and what one would do. There's a lot of rules and laws about faeries appearing behind ones threshold, and faeries are notorious lawyers about these sorts of things. Honestly, I do wonder if a Faerie would even try to be so gauche as to get behind a threshold or do something untoward to a person behind their threshold. Only the most powerful probably could attempt it (remember, Butters was safe in Murphy'so house) and it just seems like something that one does not do.
Last - Titania, cover the costs at the train station? Not likely. Though I think that Mac might have received some sort of compensation for his ceiling fans. Everyone in the Dresdenverse loves Mac.