Nice job by the author creating parallels between our two couples. Kanna wanted to pursue her dreams, Joe couldn't take the risk citing Mitsuki as his excuse. Their life goals conflicted with their romantic goals.
Now we have the inverse, where Mitsuki wants to go to the prom with Aya, and Aya thinks Kanna should follow her music dream.
What I think the author is cooking is using Joe and Kanna as a cautionary tale for Mitsuki. It won't be Joe who has the last talk with Kanna, but Mitsuki. Kanna will tell her how she loved them but couldn't stay in Japan. Mitsuki will see the bigger regret was leaving romance aside. It probably culminates in a dramatic "Mitsuki shows up at the prom and gets the last dance with Aya, (then they make out. Ok maybe wishing for too much there).
Don't let the record label take you out to lunch
You're the one who has to pay at the end of the day
-Jeffrey Lewis