As far as I know, the game engine does not have script functions that allow the casting of spells from equipped items. I recently reran tests to confirm this behavior (trying various work arounds) and the rules seem to be:
No casting for items that can be equipped, determined by base item type. Change the base item type to wand and the same item works from inventory.
For magic items that can not be equipped, again determined by base item type (wand, magic rod, miscellaneous items) must have at least one charge to work. Wand of magic missiles with charges – works. Scabbard of Blessing (miscellaneous item with uses per day of aid and bless) does not work unless one charge is added to it. The AI could automatically add one charge to such items, but when examined the single charge is shown (no workaround as far as I know). Could be done as a global option.
I have not tested but scrolls and potions probably only work as single charge spells. A protection from alignment scroll will not work because the castable sub spells are not returned.
There is an AI global setting that “allows” monsters to use items that can be equipped. What it does is find equipped items with castable spells and copy the spellcasting properties onto a newly created wand. The wand is given the same name and made non-droppable so it appears as if the item is being used in the combat log and the wand will not show up on the corpse.
Note: NWN EE added some functions that might allow the item casting.
A progress update on the next release
Because of issues with cleric (cure/inflict) and druid (summoning) spontaneous casting, the talent based system will still be used for most spellcasters. A change should allow sub spells to be more reliably returned especially the element protection spells that have five sub spell variations each. Warlocks will use the spontaneous spellcasting system because it will now use the correct spell level and does not have the conflict with cleric and druid spontaneous casting.
I put in a workaround to fix a bug in the game - spell resistance on items only working once. An effect is created for the highest spell resistance item found. Also, the icon that would normally show for the effect by the portrait is removed. This workaround includes various monsters that have the spell resistance on the creature hide (dragons, devils, demons, etc.).
I found out how to cast clerical domain spells but it would be difficult to implement in the AI.
Currently working on scheme that will allow dragons to be set up as sorcerers in the toolset but allowing the clerical spells that some types are allowed to be cast as special abilities. The clerical spell uses would be determined by the corresponding sorcerer spell level uses left. This is an extension of the spontaneous metamagic casting system done for 2.4. As part of testing, the very old red dragon creature template in the toolset had various issues that were fixed – wrong spell resistance, saves, etc. Might also give it some more feats to make up for all the standard ones omitted because they are not implemented in NWN2.