1) If Dvalin doesn't vote at all, it stays where it is now, a tie. Not sure what the moot's rules say then, since the rule was to bring in the lesser deities if there was a tie among the greater. But I'm guessing that neither a stalemate nor a re-vote would end in Hel's favor.
2) Hel's plan hinged on its speed - i.e. ending the world before the dwarves could realize they were doomed. Remove that surprise from the equation and now they have time to plan out honorable deaths for themselves - say, by storming the location of the final Gate (conveniently located near their homeland) en masse. If they win, the world is saved, if they lose, their souls are saved from Hel, meaning they still win. It might not be enough to save all of them, but probably would save enough of them that Hel wouldn't be able to wrest control of the worldbuilding from Odin.