If it has slack to slip on the crank when turning anti clockwise, then what do you think happens when you turn the belt clockwise? The slack moves to the other side. Therefore, it would be far too loose.
Yes the 'engine' is designed to rotate one way but no the belt is not. The belt has a rounded tooth design and it doesn't matter which way you rotate it, it will have equal possibility of slipping either way.
Also the tension on the belt does not change, if you rotate it clockwise, the tension is on the idler side, if you rotate it anti-clockwise, the exact same tension is on the tensioner side.