Great story. Great reading. But one glaring logic error stuck in my mind the whole time. Doesn't seem fixable.
If the "fast ship" takes 15 days of ship time, and 15 years of "normal time", then the "slow ship" should take even longer. Not 2 years of ship time, and still 2 years of normal time. I haven't done the actual calculations, but off the top of my head, I'm guessing the slow ship should take something like 2 million years.
Or else, the distance isn't that great, and the fast ship will take only like a month, normal time.
The time difference is so central to the story, attempting to fix this underlying error would screw up the whole story for sure.