I think we can summarize the whole plan as "ten million souls! Big power-up! Huzzah!"

If Hel thinks that getting 10 million dwarven souls will give her that much of an edge, and the other gods think so too, then they do.

Maybe a dwarf soul is worth 10 other souls because of its hardihood, strength, and two ectoplasmic livers.

Whatever the case, she's figured out a plan that works. Either the numbers are big enough, or the souls have special properties, or it's an ideal time of the millenium to gather soul power, or whatever. Maybe 10 million dwarves going to one god will create a soul vortex that pulls in other souls, also.

There are a million possibilities -- if the imagination amends.