Responses
I don't think so why would she write a letter to inform someone she already state she won't abandon and who knows she loves him...it more like she did send it to her father to explain that she did fell in love with someone she didn't have but was happy" the last sentence means she did stay with shinpei...like she chooses her happiness over family duties...and her father was probably sad but happy knowing was did find happiness after all even with that little time she had...my theory
i believe the old man in the beginning may likely be Shinpei because of the fireflies surrounding him and the letter as he reads it. I think Satoko did end up living a long life with him, but she died before he did. That's just how i'm seeing it tho...so they still had their happy ending even tho it's bittersweet..