Dwarves do have surface settlements, especially in their golden era, and this was the era where elves and dwarves worked together. A dwarven settlement with an elven waystone is not inconceivable, especially because, as you said, a dwarven waystone wouldn't have worked.A carved-out mountain. It's open-air and would have exposed any dwarfs not living underground to the Winds. It's closer to Talabheim than to a regular dwarf Karak.
Unless you assume that Ulric really did fucking bring down his fist to flatten out Middenheim as his myths say (edit: or possibly an elven superweapon did it), it's a bad fit for dwarfs to actually live in.
I was under the impression that Ghumzul was composed of one specific mountain and not the whole of the Middle Mountains.
And, again, Middenheim's nexus still works and transmits energy to Tor Lithanel, when we know that Karak-Waystones a) need a chain of mountains to transmit their energy and b) need a population of dwarfs to keep the mechanisms functioning long-term.