You know what? I don't think Spellcasting uses up AP at all. Only movement (and perhaps attacking) does AFAIK.

The Double AP seems to only matter for movement and perhaps a % is taken off for each swing, but I believe the units move uniformly by turn. They have a movement phase then an action phase. And an action can be an attack or a spell.

The double attack only comes from the Quickness spell AFAIK. There must be a flag that says "this unit gets double attacks or 2 spells". With the HoF quickness it increases as you stay in the HoF and there is a certain point where it gives you an additional attack/spell above and beyond 2 (I don't know exactly).

As for him castin' fire spells instead of smiting, that might be because of his range. If you have him set for Smitex5, cast spells. He'll stay in one place like a good boy and cast spells as the enemy comes in range, though sometimes he'll do a buff first (probably Fire Shield).