Just watched it. It had been so long that I forgot they never got (around) to the middle-schooler's problem. Great continuation of the series and ties everything up as well as I could have asked for.

As for how she was saved, I think the implication is that Mai, due to a reverb dream from the 'fake' present, was compelled to take the film role of a character awaiting a donor, which put organ donation into the public eye, and someone else who became a donor due to this influence gave her their heart upon their death.

Do watch. It is a compelling movie that expects you to have seen the series.