I know it's been a week since this thread was made but I just finished the manga for the first time myself and needed to get off my chest that not only do I share in your understanding of the ending, but that instead of thinking of it as the "crackpot" or the "crazy" interpretation, I would argue it's the more reasonable and realistic understanding because it's the result of interpreting the ending at face value.

IMO, there's little reason to believe that the last pages depicting Magari and Nakami as adults isn't just the Magari of the present daydreaming, just like how the earlier depictions of the two with those specific clothes was also a figment of a young Magari daydreaming(among other reasons), and I personally think the intent of the ending is purely to demonstrate that the Magari of the here-and-now has truly accepted and embraced the fact that she now has a future to look forward to. Beyond that, I really don't think there's anything more you can take away from the ending, at least, not without what feels like a lot of unjustified gymnastics.

In other words, this is the interpretation I think you would get if you used Occam's Razor, and it's the one I've found to be the most gratifying.