Well, I re-read it, and now neither ending seems so tragic anymore. I now think that there is utility in transforming the babyeaters that I didn't see before.

That said, the way they went about their supernova operation seems illogical, particularly the part about giving them 3 hours and 41 minutes. I would imagine they decided on that amount of time by estimating the chance of the superhappies showing up as more time passes times the disutility of them stopping the operation, vs the number of humans that will be killed by the supernova as more time passes, and choosing optimal time.

It seems like relatively few humans are able to escape prior to around the 8 hour mark, and, given that the superhappies gave no indications of when, if ever, they would follow (before their operation with the babyeaters was finished), the best times to blow up the star would be either immediately, if the chance of the superhappies showing up is judged to be high, or the disutility of transforming all the humans is high (relatively speaking), or they would wait 8 hours and save most of the people on the planet. Waiting about half that time seems to be accepting a significant risk that the superhappies would show up, for not much gain, while waiting about another 4 hours seems to be about the same risk again, for a much larger gain.

Still though, a very good story. I expect I'll continue to stretch my mind now and then contemplating it.