I simply can't wade through another 4 pages of this, so if someone already mentioned this, I apologize.
If we do assume some sort of flammable liquid (aka Moisture) for HW to affect is within a Fire Elemental (and I do not, Elemental Fire is pure fire that burns without fuel or air, read the EPoF description), then when HW evaporates it, it will become a powerful explosive, and turn any Fire El into a fuel-air bomb. The detonation of an elemental in this manner would certainly kill it, and also anything within a range of HD x 10 feet, no save, unless it was immune to both fire and impact, or of divine status (and even then its avatar would be anhillated). Out to 4x that distance (40xHD) a Reflex save for half of HDd100, and then drop it to d20s for double that, and d6s for double that. (Double distance, quarter damage) One last ring of d2 heat and wind subdual is the DM's option to exercise.
So, either we go by the rules' description of elementals, and leave science out of magic, or we add ALL the science, and not just what's convienent to the PCs.
I would suggest that we just stick with magic, okay? Your DM made an excellent call, but yes, he should have given you a Spellcraft or Knowledge: The PLanes roll to avoid wasting the action. Well, that or he could have agreed with you and made it a TPK. That would have been fine too.