I was just thinking the same thing! Like he will let her go or smtg, especially since the first scene appears to be on the island so my guess is that he never left and in this chapter their plan were simply too dreamlike and frankly a bit foolish especially considering her health. My theory is that he stays, she goes and she probably dies not too long after.
When I saw that scene in the first ch, I thought it was a old woman holding a letter but as the story went I thought it the old version of ml reading the last letter before she died and that they lived for some time together happily as married couples. But now reading ur theory I will have to see the first ch again. And if what u r saying is true dame i will not stop crying for ml since he deserves to be happy and away from that miserable place.
Theory time,
The sad ending is a given, but, I don't believe they will die, I think that the island is their bubble, where they believe they can end up together and happy and in love, but the harsh reality is another. He is a killer, she is an heir, her family wouldn't approve, hell, the world wouldn't, not in those times, in these times? a great MAYBE.
The letter, that that person is holding, may be their last goodbye, the last part of their love story, and the bubble will explode.
I think, that: "someone has to leave first. This is a very old story. There is no other version of this story."