in fact, barathum only talks about setting AI-guided player slots to be filles with a computer player. even without AI, a computer player on a slot will activate a rudimentary ai (The AI of Neutral Hostile without the follow target only x seconds or y meters) wich will cast spells if it thinks its neccesary (for example, multi-target or aoe spells are casted on groups of enemies, nuke spells like death coil are cast on the foe with the highest health). However, this will only work if the spells still have mainly the same effect as the original spells. So if you base all spells on channel, the ai wont do anything, because channel has no effect.
PS: summon treant is a very 'complicated' spell, it could be that the ai never sees an opportunity to cast it.
PSS: excuse me if i sound rude, but have you checked the mana costs? a footman has no mana, so he may not be able to cast.