[Kuruharan]

I think you might be placing Dáin's place in the line of Durin at far more distaff position than it properly occupies. He was son of Náin, who was son of Grór, who was the son of King Dáin I, who ruled in the Grey Mountains--and since Thrór was the elder brother of Grór, Dáin's lineage is the same as Thorin's before that. Indeed, if Dáin's family line had diverged from Thorin's before the fall of Moria, then Balin, Dwalin, Óin, Glóin, and Gimli (at least) would have had prior claims on the Kingdom Under the Mountain.

I'm almost positive that "Dwarves and Men" says that the Longbeards colonised the Iron Hills (and most of the Grey and Misty Mountains between there and Moria) long before the fall of Khazad-dûm, but Dáin's patrilineal ancestors could not have lived there before his grandfather, because it wasn't until Grór that his ancestors weren't Kings of Durin's line.

So... while I agree that Dáin's family (but only Dáin's family insofar as it is distinct from Thorin Oakenshield's) was never associated with Erebor, and while it is true that the Longbeards were associated with the Iron Hills almost since time immemorial, Dáin's family history there doesn't go back QUITE as far as that.

That is quite correct. Thank you for setting me straight on that.

Looking more closely at it, I wonder if the Longbeards, being the people of Durin, might not have been the only Dwarves to have a "king", and maybe that weighed on them all the more heavily when Khazad-dûm, and later Erebor, were lost.

-and-

So, maybe actual Dwarven kingship was reserved for those of the line of Durin

Very interesting theory. I can't find any textual evidence against it although I'm not inclined to agree with it.