Visa requirements
 

I'm a US resident planning on booking a RTW starting in Japan combined with a Japan round trip. The idea is to fly out to Japan on the round trip, use the RTW to return to the US via Europe, then do another trip to Japan in six months on the RTW, then use the return portion of the first ticket.

My question is whether UA will deny boarding on the first leg since I won't have a Japanese residency visa. I have already confirmed with the Japanese consulate that this itinerary is allowed.

I guess worst case is I bring a letter from the consulate with me to the airport?

I'm missing something. Why would you need a Japan residency visa?

As long as you have a ticket showing you are departing Japan within the visa-exempt
period, you will have no problems whatsoever.

And there might be the problem. OP is flying to Japan on return ticket with return in 6 months. The fact that OP has another RTW ticket starting in Japan and ending in Japan in 6 months again will not probably help the case...

Doesn't OP just need to show proof of onward flight within 90 days?

I have done similar ( or maybe worse) - gone to NRT on a one-way ticket, then started
a RTW from there, returning much later.

As long as you show that you have a ticket to depart within the visa-exempt period,
there will be no issues whatsoever.

And likely the only person asking about it will be the check-in agent for your first
flight; as long as you show them you have a reservation to depart, there will be no issue.

Thanks all! Looking forward to these trips. Amazing that RTW fares are so much cheaper from Japan.