Balrog Breaking and Entering

[Legate of Amon Lanc]

But at the time when he was supposed to get there, I would assume the doors were also guarded? I mean, even though the door was open, you would have a Dwarf or two sitting there, if nothing else, and it would be somewhat awkward for the Balrog to get through customs.

As for the Grey Mountains/Withered Heath question, well, maybe that's just it - without the central leadership, maybe the Balrog did not feel as comfortable to share his place with some dragons. No, it would be somewhat "let's stay each in our own territory, here be dragons, here be spiders, here be balrogs". Also, what do we know, maybe Durin's Bane had a special affinity for mithril?

And one more note to the Grey Mountains - it's not that they were totally left alone either. Dwarves have been there at some point too, and possibly (maybe) already Men on the southern side (the Northerners lived there certainly later, resp. then the ancestors of Rohirrim). And Orcs (at least around Gundabad, so probably outreaching to the western part of the Grey Mountains).