(Or, the prone shooter rule strikes again).
Mythic elemental body spells have a useless addition to them.
...There is a 50% chance any critical hit or sneak attack against you is treated as a normal hit, as if you were wearing medium fortification armor.
However, the elemental subtype grants
Not subject to critical hits or flanking. Does not take additional damage from precision-based attacks, such as sneak attack.
It's correct.
Elemental Body nor Mythic Elemental body grant you the elemental subtype as far as I can tell, so you don't the elemental subtype abilities. Now, it's not useful for elemental body 3/4 which specifically call you out as having immunity to critical hits, but that's slightly different.
Elemental body doesn't give the elemental subtype. For the first two of the spells, it doesn't normally protect against crits and sneak attacks at all. Three and Four don't protect against non-sneak precision damage at all, actually.
Also, there is still some benefit to the fortification effect: X Burst weapons (and by extension, possibly the Critical line of feats) still activate on crits, even if the target creature is immune. So Mythic Elemental Body III or IV makes you more immune to crits than regular elementals!
That is... entirely unintuitive. Another one for the house rules....
That is... entirely unintuitive. Another one for the house rules....
Not really. None of the polymorph spells change your subtype, and it's done on purpose to prevent the spells from being overly good.