Would it make sense for OP to show a (refundable, possibly award) ticket for the Japan-Korea RT? Will OP need a visa for Korea?

BTW, if Japan is doing single entry business visas, would the OP need to enter Japan in order to connect back from South Korea on separate tickets? I would assume that luggage cannot be checked through, but of course this depends on the airline(s) involved. My understanding is that Japan does allow transit on separate tickets, but you cannot plan to stay overnight at NRT nor to transfer between NRT and HND.