The Spanish flu didn't kill off "20% of the world's population", it killed off 3%. Sofia spent 50+ years locked up and can't be bothered to spend 1 minute on Google?
Maybe it's an Alternate universe.
Alright, I'm confused. The major driver of the plot in the back half is the aliens realization that their world was a couple months from getting fried by their sun, thereby necessitating bringing them all to earth and the elimination of at least a corresponding amount of earth's human population. Disregarding the highly increased crop yields their more advanced technology would probably be able to produce, okay. But then they bring through their entire planet... thereby negating that whole point, right? So why did they go through all that if they knew the entire planet was coming?
Problem is, the alien planet looked like it had already been burned up by the supernova and isn't habitable anymore. If there even are any survivors on there, they likely need to evacuate to Earth.
If they do indeed intend to move to Earth from their planet, why don't they just use the sufficiently advanced tech they have and terraform Mars, or Venus instead?
We don't really know if they have that kind of capability. Terraforming is a pretty big job.
What makes it worse is that they're bringing a billion aliens. There would be some starvation and resource problems if the world's population suddenly grew by a billion humanoids, but they shouldn't need to kill three billion humans to make room for them.
If the aliens are immune to the Spanish Flu, and they needed to infect the mall anyway, why is there a timer on the device to spread the disease? Usually one puts a timer on a device if it's dangerous to you and you need to get away.
Probably habit, since most bombs on TV shows have timers, or necessary to drive the plot and give the heroes time to stop the explosion (though it happened with another bomb anyway, so...)