Zook does not currently appear to have 8 ranks in knowledge the planes, the feat energy substitution, nor a summon spell, though that may mean a spell from the (summoning) subschool or a spell with summon in its name...
The contact with an Elemental line seems a bit open, it doesn't seem to require more than being present for a casting of a summon monster spell to ask a fire elemental to light a campfire. Although a DM could potentially demand that the event not take place during combat and may further require that the elemental be present in a more manifest sense, such as from a (calling) spell, though I am not seeing it in the description. Also, starting at level 10 leaves lots of room to include such encounters prior to joining the campaign...
Speak language is one of the rarest skills in the game, I would be tempted to limit it to languages spoken by creatures that can be identified using knowledge(arcana) or knowledge(the planes) for flavour reasons, but considering that wizards can learn comprehend languages and tongues, it doesn't look like any sort of issue for balance. Scry also has been a very rare skill since, well, Spell focus granted +2 instead of +1...
It feels as though you should be prohibited from casting spells with the [opposed element] subschool, but it only seems to prevent using energy substitution(opposed element). The whole class keeps feeling like the wording is off.
Immunity to paralysis(level4) and stunning(level 7) are something to watch, but not overwhelming by themselves.
I would be tempted to replace "any spell using that type of energy" in the energy focus and energy penetration abilities with "any spell with the [chosen element] subschool" and specify that the energy substitution feat inserts the relevant subschool. The original wording just seem to invite arguments over flavour text and spells with unusual damage effects(prismatic spray for example).
The loss of spellcasting and damage vulnerability makes level 10 an unlikely choice from a powergaming perspective. but for what it is worth:
Immunity to critical hits, flanking, and "person" spells is a lot, but not that uncommon at these levels.
50 foot move speed is interesting, but shouldn't be that much of a factor for a spellcaster unless they are running away from the party, and the D.M. will eventually get melee brutes with teleport and reach...
Those are the stats for fire elemental, The other elementals are a bit more worrying. Air can move extremely well, and whirlwind cheaply creates a cloud that disrupts spellcasting and block line-of-sight, it also allows movement without provoking AoOs. Earth elementals can move freely through earth, with obvious implications for blocking line of effect and bypassing dungeons, and water elementals may be able to dispel any [fire] spell with them included in the target, depending upon how you read the drench ability. But assuming that Zook
Flamespit takes fire savant there shouldn't be any issues.
As their appearance changes you probably want to establish some mechanism for NPCs to become aware of the properties of the new form as it seems to suggest that only other elementals will notice anything...
The wording on this is just foul "She also takes double damage from energy attacks allied with her opposing element unless the attack allows a saving throw for half damage, in which case she takes half damage even on a successful save. " If you get a save for half damage you take half damage on a successful save? I think what they are saying is double damage unless you make a save, in which case damage is half normal as is normal for a successful save. I would just add a multiple, or 100%, to the damage, multiples are a bit tricky with immunity as half(50%) is worth about the same as double(200%) in many respects and you have to account for doubling twice, such as from a critical hit from an elemental spell. I would probably go for: More immunity than is necessary for 0% = add one multiplier to damage; 0% damage = 50%; 50%=100%; 100%=200%; 200%=300%...
DC 26 is high for an ability, but 1d6 burn is not much at high levels.