I've been flying extensively for 20 years, lived in Japan for over 10 years, flew both JL and NH many dozens of times, I NEVER put my middle name on a ticket unless it's issued by United Mileage Plus because they have it in my profile, even then it often doesn't print in its entirety depending on airline. All tickets I ever buy myself do NOT have my middle name and I've never once had an issue.

All that BS that they try to feed you in the US about your full name being required is exactly that, just BS. First name and Last name are sufficient anywhere in the world. Immigration in most countries (Japan included) doesn't even look at your boarding pass, they could care less what's on it.