[Arralen]

A mage does not move over to other mages ... in fact he doesn't even know about them. He moved over behind the rearmost squad, as he was ordered to do!

Never, I mean NEVER set a mage to "stay behind troops" !!

A commander set to "stay behind troops" will move to the rear of the rearmost non-fleeing, non-bodyguard squad and stay there until the squad flees - but nothing more. No matter if he's a mage or a priest or has spellcasting ability from an item, he wouldn't cast any spells.(or would attack an enemy unit in melee, unless attacked himself)

But you want your mages to cast spells. Therefore their script should read:
"cast [spell XY]"
"cast [spell XY]"
"cast [spell XY]" OR "attack -" if you want him to move up to the front line
"cast [spell XY]"
"cast spells"

Be careful to choose your spells XY in such a way that the mage will be able to cast them.

If you want your mage to attack after buffing up, make sure there's a way for him to reach enemy forces. "attack nearest" might be a bad idea after 4 rounds of spellcasting - you troops will be engaged already as "hold and attack" has them on the move on the 3rd turn. With troops engaged and squads fanning out he won't be able to find his way around them and get stuck in their rear.
Therefore, use "attack nearest" only on turn 3 but not later, or place your troops and mage/pretender in such a way that there's a clear path for him towards the enemy. If he needs 3 or 4 turns to buff up, better set him to the side and use "attack rearmost" or "attack archers"... . Just make sure he's strong enough or assign him some bodyguards.