I agree that somehow they portrayed Alexander as way too simple. A great character, with charisma, but you could still consider some kind of idiot... that is for now.
After all, being able to get Gilgamesh, Berserker, Lancer's master out is already a very smart move. He even got Saber's true name in the process. You can consider that a great achievement.
Then, he was able to force Lancer's master to let go of a command spell.
He's been able to gauge skills of Berserker, Gilgamesh, Saber and Lancer.

Quite a great/bold move getting him good results IMHO