Thanks for the questions! I've had to mull over two of them a bit, and here's what I meant when I wrote them.
Does the Elemental Turning work only on the four domains required of the Elemenal Theurge, or is Plant also included (since you can rebuke/command plant creatures)?
It only works for the turning and rebuking of elemental creatures granted by the domain abilities of the air, earth, fire or water domains
Also, for the spell requirements: are the elemental descriptors for 2nd level spells, or all spells?
Wow. That requirement is certainly badly written, isn't it? On re-reading it, I would say:
Spells: Able to cast level 2 arcane and divine spells. Able to cast at least 3 arcane spells and 3 divine spells with elemental descriptors such as [acid], [air], [cold], [earth], [electricity], [fire], [sonic] and [water].
Is this ability for all elemental domains the character has, or only one that they must choose? The wording is in singular, that's why I'm asking.
Typically, you are going to be hard-pressed to find a deity that has more than one elemental domain, and a druid using the optional rules can only have one elemental domain.
The goal behind the ability is to improve the character's level for turning checks suited to the element in question, so it would only apply to elemental domains... Although unlikely that you may have two elemental domain, it could happen and thus it would indeed improve the turning / rebuking ability for both.