Balin, Moria and the King Under the Mountain

[Kuruharan]

Dain was and would have remained King of Durin's Folk even had Balin succeeded. He was the one with the better bloodline. It was impossible for Balin to upstage Dain in this way. So, Dain actually could have chosen to return to the Iron Hills and left Balin as King Under the Mountain and Dain would still have been King of Durin's Folk. However, since the Iron Hills were rather poor Dain went where the real money was.

Where you ruled did not matter that much. It was whose son you were.

Thorin would have been [ had he himself lived long enough to be crowned as lord of one of the traditional [although not original] homes of the Dwarves.

I believe (not totally sure) that Thorin was King of Durin's Folk anyway. He was just without a capital. He was a king in exile, but still a king.

If you think Dain, would he have claimed lordship over MOria and come to dwell there?

Well, actually, even if Balin had succeeded, he still would have been subordinated to Dain since Dain was still King of Durin's Folk. As a loyal member of said folk, Balin had a duty to the "head of the household." So the answer is "yes and no." Yes, Dain would have had a degree of lordship over Moria since he was still the over-king. No, its unlikely that he would go there himself to take over. He had other things to be doing.

The position of King of Durin's Folk probably equates well with the High King of the Noldor during the First Age. The king is the leader of all the Longbeards, but this rule tends to be rather nominal in realms other than where the king personally is. Tolkien seems to have had a real thing for this type of political system.

So, I think that Gloin gave an accurate representation of the motives of Balin in going to Moria. He was trying to give his people, the Longbeards, a wider place in the world and access to more resources. Dain was reluctant to let them go, not because he was worried about being upstaged, but because he was worried about Durin's Bane killing the lot of them.