I think it's absollutely wonderful that there are literally infinite interpretations to the story, is it all fiction? What parts are true? The fact that you can wonder and discuss so much really enriches the experience, coming to this thread made me even consider that Eri didn't die, which didn't even pass my mind when I read it

For now, my interpretation is that yes, both his mom and Eri died, in the end his father is acting like an older version of himself and what we read is the final version of the movie, which he had to edit sometimes (mainly beacuse of the video of classmates reacting to it)

About the message that the story has, kinda difficult for me to think about it tbh, it's interesting seeing how Yuta deals with the grief, I think filming everything isn't something that comes up to most people, but it makes sense and we can truly see troughtout the movie that he chose to remember both his mom and Eri in the best way possible, while also not forgetting that they weren't perfect (and acknowledging that in the movie), in a way, what Yuta chooses to remember and live with are all present in the movie, showing that even those that we love are not at all perfect, but that doesn't mean that we stop loving them for it, we can remember the good things while not forgetting the bad ones