Demons don't need more than one turn, so there's no need to hide them. Scrolls and tomes will appear if you're lucky. For mass speed boosts, you need to spend a turn, and therefore you can't use a barrage of arrows with the wasp swarm, which is the basis of your entire tactic.

The demons won't take all the mana. They have less initiative than the elven units. Not all demons will take mana, but only those that remain. If you don't hide the demons, they will be torn apart. Dragons have an initiative of 14, and fairies have 14 or 15. Others also have a decent initiative. After the speed boost, the demons will only move after the archers, dancers, unicorns, dragons, and fairies. In my elf build, I have +1 speed, and attack and leadership are upgraded. Do you think anything will remain of the demons if they are not hidden? :)

The chances of getting control over the enemy are also not 100%. They also need to build a level 5 mage guild, and they also need to be lucky enough to have a spell there. I have enough time to get scrolls.