This is you projecting a bunch of stuff that isn't presented in the manga. You're projecting that Mitsuki is aware of the reason for the split, and that she is thankful for what Joe did.
But we haven't seen any of that.
We need to go by what is actually presented. And since Joe and Kanna split precisely because of Mitsuki, then whether Mitsuki knows and appreciates this is completely core to the Joe/Kanna/Mitsuki relationship, and if Mitsuki does know/appreciate, then that needs to be shown. The fact that it's not shown indicates that the author is dismissing that.
And that's where we get to narcissism. By omitting that crucial core, the author has set up Mitsuki as self-absorbed. Then we have the fact that almost everything exists to bolster Mitsuki's going her own way and getting applauded for it. And it's a similar story for Kanna who went off and did her own thing, but now is laying the blame for the cost of her decision on Joe.
Sorry for the essay but you said I'm projecting and I'm obligated to provide proof. Here's what's been shown in-story:
- Mitsuki put two and two together: She was in the room when Joe/Kanna had their split, and in the current time Joe literally says to Mitsuki that he was convinced he had to go to America, but chickened out from going overseas to play music. He thought family was too precious, but has no regrets. But he would never forget the look on Kanna's face.
- Mitsuki and Joe are close: She's comfortable enough with Joe to wear clothes she likes at work/home, but wears a skirt at school because of childhood trauma and "hides herself" per Aya. She has intense social anxiety at school but not at home. Childhood memories and photos show the two thick as thieves and nothing's been shown to indicate anything's changed. She doesn't show any signs of distance or avoidance with Joe. She works at her uncle's shabby CD store, not somewhere else, despite getting ditched by him more than once. She hangs out with him outside work. She immediately hands her phone to Joe so he can see a personalized playlist from Aya. Their tattoo timeskip pic. When Mitsuki says she's watching a movie it's not even a question whether they'll watch together.
- Mitsuki is low-key grateful: The same way a bio child isn't assumed to be self-absorbed because they don't profess literal words of affection to their parent, Mitsuki shows it by behaving, communicating, sharing moments, and respecting Joe. Love languages are why people are okay not hearing outright words every day, because these actions are a way to affirm those feelings. Mitsuki mostly shows affection for her uncle with quality time, acts of service, and some appreciation. She listens to Joe when he says to do something. She entertains Joe's antics without complaint to the point of sickness without knowing why. On first ask about what's wrong, she brings up Aya. Joe regularly asks Mitsuki to invite friends over, and as soon as she makes friends she brings Aya and Narita. She gets Aya to help out at the store. The one time she gets scolded for staying out and not calling, she sincerely apologizes. The one time she directly asks Joe for help, she does so with a deep bow. Mitsuki says she'll make sure to pay Joe back after buying her a guitar. Joe has called Mitsuki a dumbass and a dunce with zero negative reaction from her.
- Mitsuki can be a kid: Mitsuki can learn/play guitar, write music, watch movies with Joe, and go to concerts/fests with Joe. No chiding to study or stop playing with friends. In fact he's happy Aya got Mitsuki out of her shell. She can get a tattoo, which is way more prejudiced in Japan than the west. Multiple piercings, which Aya's friends call bold before freaking out about her tattoo.
- Mitsuki sometimes thinks in black and white: Through Joe's multiple statements that the split and recent argument has been his fault and Mitsuki's witnessing of the breakup, she is telling Joe to go to the airport. She criticizes him for running and believes he should go after Kanna without hearing Kanna's side. In favor of closeness, just like Joe calling her a dunce and a dumbass while giving his thoughts, Mitsuki is doing the same for what she believes.
As I said before, narcissism is an escalation that suggests outside intervention is needed to correct. The focus is on her by default being one of two main characters. She did get criticized before, which is why she wore a skirt and was a loner and hid her personality until she befriended Aya in high school. Things only started going well after that. What does getting applauded have to do with anything? Like I said before, playing music publicly is a trope to show that she has a talent for it, that her singing and music isn't just good in her head. This is common sense; if a musician is going to turn their hobby into a career they have to find out whether their music draws people and if they have the grit to perform in front of an audience. None of the applauding did anything to change her, she was actually miserable after her singing went viral and she became the center of attention. Her social anxiety was so crippling that she was panicking because of
crutches. At one point her labeling fears were visually pinned on her: Wannabe, Poet, English Song, Boyish, Showoff, Split Personality. When her lyrics were leaked, Mitsuki was frozen, surrounded, and crying in the classroom until Aya stepped in. All of her leaps in confidence have been from the support she got from her friends, with Aya leading the charge. And Mitsuki didn't even mesh well into Aya's group without a big move.
Between Mitsuki and Aya, Mitsuki clearly has more to fix. Aya's nearly emotionally self-sufficient aside from insecurities about a friendship that isn't progressing to romantic. Confiding in Mitsuki about this is both the problem and solution. She's also Mitsuki's oshi. Fans support their objects of affection and the idols keep doing what they do, which is what fans want. And Mitsuki stuns. Mitsuki sang to Aya. She then went to Aya first to sing her first song, her biggest passion and something she's so sensitive over that she went catatonic when exposed. Most of her rizzing is her doing something for Aya or complimenting her. Mitsuki hugged Aya in tears while thanking Aya for getting angry for her. There's more, these are just the details I picked up from searching Joe chapters.
In a fictional story, everything is deliberate. If we can't infer affection and kindness from the examples given, then why did the mangaka put it in the story? There's a reason why most stories with family don't regularly affirm affection with an "I love you" and it's not because they don't love each other. It takes the literary theory out of the equation and makes for a very bland tale.
If we go purely by what we've been shown, then my assessment of Kanna is incorrect and there's plenty of validity in the pro-Kanna camp. Kanna and Joe had a plan to go to America together. Kanna continued with the original plan, so rather than Kanna going off and doing her own thing,
it was actually Joe who went off and did his own thing. Joe blames himself. If Kanna blames Joe then this gives her a reason to be angry and Joe a reason to feel guilty. No shows of closure so we can't say the issue was laid to rest. Kanna says she's back for Mitsuki, so she has a reason to be present. She actually meets Mitsuki 1st and her 2nd appearance where she runs into Joe is to take Mitsuki out. 3rd time is again Kanna and Mitsuki before Joe walks in and Kanna exits. It's only the 4th time when they're together in the store, and it gets stated that Joe hires Kanna as a backup part timer when Mitsuki's not around. We aren't shown proof Kanna is playing games or jerking Joe around so we can't say that's the play. In fact, before chapter 109 there have only been 2 cutting remarks from Kanna, both were preceded by Joe bringing up a reference that was tied to Kanna's departure or his younger self. The first was a true cutting remark, the second was using his life experience to show he should let mitsuki make her own choices.
And I just realized on the reread that back when Joe said his realization upon choosing to stay was that he had too many precious things in his life like family and he'd never forget the look on Kanna's face, that's what she was thinking about in chapter 109 when she says she wanted to be family with Joe! She says Joe didn't wish for them to be family because
his reason for staying was family, and that definition of family didn't include Kanna! Oh wow, kapoof.