Very powerful - so emotional! I can imagine why Lois would keep writing the letters, but then immediately destroy them...after so many years, where do you even begin to hold this conversation? I actually thought it was going to end like your first ending - there's a finality to it. The story feels more substantial without a happy ending, though keeping the letter gives just a glimmer of hope, a tangible reminder than he has something to hold onto.
Not to bring up yet another discussion about deathfic, but I actually wasn't bothered by it in this case. I've thought about why all day and have concluded that it's because, in the story, she's been presented as dead from the very beginning. When she starts out alive, I hold onto the hope for a miracle or a happy ending where she lives, and them I'm sad when she doesn't. Here, I could just focus on the story from a hopeless position already, which made me feel more for the struggle of both characters. So, kudos to you!
