[Tuor in Gondolin]

I don't think you're supposed to fully know what the threats to Bree were. As JRRT says in one of his Letters (something like), it's the hint of barely glimpsed vistas that give verisimilitude to a tale and a created world. Even PJ's movie uses this concept to good effect in the beginning of FOTR when Bilbo says:

Middle-earth being, after all, full of strange creatures beyond count

and then immediately moving on to discuss prosaic hobbit life.

Not unlike some other Tolkien throwaway comments, like the one about hobbits wandering off to have adventures and (I think) some never returning.

And is it that hard to freeze Butterbur's heart? Regular brigands might do so, but I'm thinking more of something like the giant insects, etc. that unsettled Thorin and Co. in Mirkwood.