A question has been raised in my mind - are there any dwarves outside the demesne of the Northern Pantheon? Not visitors like Durkon and Hilgya were, but actual clans doing their dwarvish thing for generations? I don't think so, but it would pose a conundrum relating to the bet.

Another question: did Thor have "standing" to make the bet? Do no other gods of the Northern Pantheon have an interest in dwarven souls? We know there was a dwarf high priest of Odin, which indicates Odin is doing the "pass my power to a cleric to gain worshipers" bit with dwarves - so, Odin must collect some of the dwarf souls that die with honor. And Loki as well. Durkon's explanation to Malack was that dwarves that died with honor go to the plane appropriate for their alignment.

It would serve Hel right if she did manage to get the world destroyed and then Loki pulled the "invalid contract" card and the rest of the Northern Pantheon backed him - because every one of them stands to lose souls they might otherwise gain, although probably not too many for Thrym and the like.