I'm a little bit surprised about the consensus forming that Mitsuki is so "untrustworthy" and lacking dedication. How serious she has been was shown in the post-coma years. Also keep in mind that in the time in which she had no damn good reason to distrust Takayuki, she was levelheaded and supportive - not clingy at all (see her dragging him to the hospital and pushing him to get close to Haruka). This only started to change once Bakayuki started the "daijobu" pretend mode, and understandably so to me.
In other words, once you're in a relationship with ANY girl, and start treating her like the way Bakayuki did, don't expect her to even hold on to you as long as Mitsuki did. You'll just plain lose her. If anything, I consider the fact that she's still connected with Takayuki on a little thread a little miracle
If the roles were reversed, and it would have fallen unto Haruka or Akane to care for Takayuki while he's off visiting his old flame Mitsuki in the hospital, visibly falling for her ... I think both Haruka and Akane would seem verrrry different too.