I have to speak up since Iwon't let Mitsuki be dragged through the mud. Mitsuki was my favorite character by far in this series, even when she let me down by succumbing to the immense pressure she was under. Still, of all the characters in the show I feel like she made the least bad decisions for the group, while still trying to make the best decisions for herself. I don't assign blame to anyone because most of the decisions made in the show where made based purely on emotions and not on thinking things through so I can't hate anyone for being a douchebag.
Takayuki made the wrong choice when Haruka woke up, it would have been better for everyone (except maybe himself) if he had stayed with Mitsuki and let Haruka's parents tell her that he had to move on while she was in the coma. It would have been hard for him since he did care about her, but in the end it would have destroyed Mitsuki and she didn't deserve to be tossed away like that.
Haruka couldn't really make the choice to give him up since everyone lied to her, but you can't live in the past. She needed to accept reality or just go back into her coma or die. If she can't face reality why should everyone have to play along in her fantasy at the expense of their own happiness. Is it nice? Yes. Does it demonstrate a very deep love? Yes. But is it an ultimately stupid and doomed course of action, causing more aggregate pain to everyone involved then just forcing her to deal with life... YES!
In taking care of Takayuki, Mitsuki started to become dependent on him. Her dealing with him seemed to help her deal with her own problems and feelings about what had happened and she was able to move past them while she was with him. When he started to leave her for Haruka she started losing her resolve and headed right down the path Takayuki took after the accident. I can't see anything that would validate Takayuki sentencing Mitsuki to the exact fate that she rescued him from, it's just too wrong. He may have felt he owed Haruka for their short relationship and for not being there to keep her from being hit by that car, but I think he owed Mitsuki his life and sanity which I feel counts more.
I think everyone in this show made poor decisions based on their emotions, but in trying to look at it from an objective standpoint I think the final outcome of Haruka letting go and Takayuki staying with Tatsuki was the best course of action.