Alright. After checking Monsters again, I would alter it slightly. The text states:
Incorporeal— Material attacks such as sword strikes and fists cannot hit the creature, only magical attacks or elemental attacks can actually harm the creature. Magically enchanted weapons can hit but do criticals two sizes smaller than their normal critical type.
So, my interpretation would be about half right. Magical weapons do damage, but their crits are reduced.
So, the Hammer strike would do the double concussion damage, so a Hammer Strike Long Sword would do a tiny critical compared to the normal medium.
When using Elemental weapon, it also does a tiny crit instead of a medium one, the extra elemental crit is not affected, though, since these type of creatures is harmed by elemental attacks.
The ruling looks pretty clear to me, though depending on your campaign, you might want to alter it. To give the players more of a chance for once, but mostly to stop players waving around with torches in ghosts, which looks silly, but is, by this rules, much more effective than using a sword. ;-)