Awakening of Evil

Regarding the mines – yes.
The Elves won't give you heroes.
Red – Demons. You need to get a quest to destroy the Inferno Castle, or something like that. I saw somewhere that the Sphinx gives the quest, but I don't remember what you need to do to get it.

And there's also an Inferno Castle at the end of the map. You'll have to fight a demon there to complete the mission.

I destroyed the Inferno first and got the Tome of Dark Magic. There's a portal on the battlefield of the Orcs and Humans for this. After choosing a side, you can use the portal and reach the Sphinx, where you'll get the quest and have to destroy it, but it's a yellow demon.

Another question about the Elves and Orcish Hunters: it seems that they both give quests. I took a quest from the Elves, went to the Orcs, and they also give me quests, either to kill the Druids. After that, a window appears asking if you want to attack the Orcs or cancel. One of the quests will definitely fail, or is there a way to complete both?

I also don't understand about the spy, Axel. I searched the whole city, and no one said anything. Only the Centaur mentioned that Axel told me some nonsense about a horseshoe. Anyway, I talked to everyone and returned to Axel, but he had no reaction