“My criticism is that this seems to be more of a seed for a series of collaborative writing prompts than a game that can be lost.“
That’s exactly right. This isn’t a game where one’s goal is to win, as in “complete a condition by the time you’re finished playing it.” It’s a collaborative story-telling (well, writing) game. The win condition is that both players are pleased with how the story turns out.
The goal is to creative a cool narrative in the letters. Whether or not Sender or Receiver can be trusted is entirely up to the characters— there aren’t conflicts built in, because the players aren’t in opposition. You’re building the story together.