One question... in the second-name scenario, if his name actually is Smith John Lastname but not John Smith Lastname as in the plan (well you never know), then it becomes really embarrassing.

If I feel thankfully about not seeing a person, it probably isn't because of the name thing. Most of the time in business, it's not a shame forgetting names of people you don't deal with / just randomly popped up in your life once or twice. Rational people do understand. Honesty outlives little smart tricks, and it is a big deal especially in the first impression. That's what I believe in. What I'd rather do is to say sorry and ask again, with a polite and humble face. Sometimes a self-deprecation might bring surprises.