I looked up 3.5e Shapechange, and it referred back to:
I then looked up Polymorph, and it referred back to:This spell functions like polymorph, except that it enables you to assume the form of any single nonunique creature (of any type) from Fine to Colossal size.
I then looked up Alter Self, and it says:This spell functions like alter self, except that you change the willing subject into another form of living creature.
Ergo...a Horned Devil has the ability to speak—known as Infernal—since it is a Greater Baatezu, yet one could say is ever more complex than the vocal needs requiring humanoid spellcastingYou retain your own ability scores. Your class and level, hit points, alignment, base attack bonus, and base save bonuses all remain the same. You retain all supernatural and spell-like special attacks and qualities of your normal form, except for those requiring a body part that the new form does not have (such as a mouth for a breath weapon or eyes for a gaze attack).
You keep all extraordinary special attacks and qualities derived from class levels, but you lose any from your normal form that are not derived from class levels.
If the new form is capable of speech, you can communicate normally. You retain any spellcasting ability you had in your original form, but the new form must be able to speak intelligibly (that is, speak a language) to use verbal components and must have limbs capable of fine manipulation to use somatic or material components.
Additionally, a Horned Devil have limbs and dexterity of equal measure to what humanoid spellcasters require.The stilted, formal, grating yet strangely melodic tongue of the greater baatezu is full of obscure patterns that seem to meander misleadingly before snapping to an unexpected point. Speaking it properly requires careful planning and the ability to respond intelligently to phrases based only on the most subtle of cues. It's said that two greater baatezu can hold entire conversations in this form of Infernal using only the beginnings of their sentences. It is nearly impossible for mortals to learn.
A Horned Devil form from Shapechange should be fully capable to cast spells, in the D&D 3.5e world.
Thus, I question the Rule against the Horned Devil form NOT being able to access the Quickcast menu, for casting, and bypassing whatever Obsidian did to stop that from happening. On the other hand, Nightwalkers communicate telepathically, so they can't cast spells that require somatic components. Obviously, Iron Golem doesn't probably have the dexterity for this. But, it is very possible that both Fire and Ice Giants forms are fully capable of casting!!
So, just because NWN2 Shapechange spell says "You cannot cast your spells while transformed. ", it don't actually mean that is the truth of it!
*rotten vegetable shields up at full*