Sacred Haven of Debauchery

No, it doesn't turn; that's what I'm saying. "YOU CAN'T SEE THE ENEMY!!!", so you have to turn so that it IS visible, even though the mob is only ten centimeters away from the character.

Actually, there's a 90-degree sector for both mages and melee units. Moreover, if a mage starts casting when the mob is within this sector, and then the mob moves out of it, the cast will still go through as intended.

90 degrees turned out to be quite small.
Either make it like in PvE – you click on a target, and all attacks will be associated with it if it's within the attack range, and if not, the character will move towards it. Or like in some slashers, where you can swing your weapon into the void as much as you want, and when a target appears, you'll hit it.
In general, PvE will be faster in terms of mechanics, but it's more grindy.