the letters were written after her supposed death and they speak of decades, so she did live. unless the entire meeting with her bodyguard whatshisname was also a delusion, which doesnt make sense to me since why would he think of his rival at a moment like that
I think more of the ending of the letter may have been a delusion but that is my own theory
If the manga goes with the logical side she wouldn't have survived
Of course it is fiction so anything could happen I just theorise this because it fits with the tones of the manga in my opinion
to add on to this he only sees her and himself as younger versions.
We only see them both old together in the last panel where they are hugging (wrinkled hands and body type) but even then we never saw her aged appearance which makes me think he couldn't imagine her older as he had only seen her when she was young and alive
first panel as well, before she turns, you can see wrinkles around her eyes. he sees her as young sakoto and he 'turns' into his younger self as well imo because its meant to portray that its like time stood still for them, they still feel about each other as they did in the past, but they're both drawn as old people. i think wanting the ending to be better than it was is fine but fr what was written is what counts
I think the fact we never saw her face until she was portrayed younger plays a part.
At the end of the day it is just my theory and opinion.
When I first originally read the last few chapters without a second thought I interpreted it as delusion as if that was what it meant my reading style was picking up on clues and shadows etc but of course others can see it for how it is written straight forwardly.
Everyone hates the ending but it is realistic.
He wasn't right in the head. When she died he tried to do good but reading over them letters and reliving the memories in the house brings out his craziness(mental illness?) and he imagines she's back and alive.
It's not real, she did die.
Gotta remember he deludes himself when it gets too much