Heimdall could just have his priest leave the room, since we've already established that no priest = no vote, or Roy wouldn't be fighting in the first place.
Assuming, that is, that Heimdall changed his mind, which isn't established.
The whole "millions of dwarf souls = unlimited POWER" thing is kind of cliched and stupid though. And mathematically illiterate. If Odin has thousands of years of dead humans, one generation of dwarves isn't going to tip any balances. I would expect Heimdall to shrug and proceed with world destruction as the prudent course.