First, he wasn't asked to pick a foreign leader he agrees with 100%; he was asked to pick a foreign leader whom he likes. Is he so much of a politician that he can't say a name without fear that he will ruffle some Libertarian feathers? Second, and by Johnson's admission, there are foreign leaders he likes but doesn't know their names, so your defense of him is moot.
The email story was hardly a disqualifying incident. The foreign leader story was, in my opinion. It shows at the very least that he's not at all prepared to be president.