If no one over the age of fifty would be selected, then the sheer number of older U.S. citizens who were either euthanized (to spare them from suffering through the horrors of the comet strikes) or given the means to commit suicide would have made the Holocaust look like a small-town Santa Claus parade.
The Messiah has an Orion Propulsion System and was assembled in space. It's way too small to be one. (Orion was a military project to build a deep space craft in the late 50's, it works by exploding nuclear bombs behind a very thick metal plate and getting hit by the explosion. Most designs are assembled on the Earth and are about 1 mile in Radius).
The Messiah was a big ship, as it needed to carry everything the astronauts would need for what's essentially a deep-space journey of roughly one year. To that end (under the heading of Crazy-Prepared) why not load several cruise missiles adapted for spaceflight and destroy the comet by using visual guidance to shatter it far out in space by hitting it with a couple to create fissures and then flying a nuke inside? Better yet, why not just tell the world, use Earth's combined resources to build a fleet of Messiah-class ships to hit the comet as mentioned above, or (just as good) have the fleet soft-land one of the vessels, lock it on the surface, and use the ship's Orion Drive to divert it by fractionally adjusting its course away from Earth?
Why did all of the people on the highway stay in the area, only to be killed there when the tsunami hit? Why didn't they all just evacuate weeks before just out of an overabundance of caution, and even if they lost their homes, they would still be alive and capable of starting new lives?