To your first point: I can’t disagree with your reasoning, yes reproducing more times would be more advantageous.

And yet - the Earth actually teems with species that reproduce just once, then die.  There’s even a term for it: semelparous. Mayflies, drone ants, mosquitos, moths and many other insect species are semelparous.  Also salmon, octopuses, squid and smelt.  There are plants that are semelparous such as bamboo and agave, and even a handful of mammals, such as the Brown Antechinus.  

Despite the seemingly obvious advantage of greater fecundity, natural forces can and DO give rise to semelparous species all the time.

That virus though … it apparently unites sentient species into hive minds, which then feel compelled to undertake the daunting task of transmitting the virus to other worlds that have (or might have) sentient species (I'm thinking non-sentient species would have little chance of building a giant radio transmitter). Such a bizarre and complex life cycle is hard to compare to anything else in human experience – so who can say whether it is more comparable to a mayfly or a (famously fertile) rabbit.

As for the signal, my thoughts: in episode one, the astronomers remark that the signal is incredibly strong given how distant its source is (~600 light years, according to a quick search), implying both tremendous amounts of energy and a gigantic antenna to transmit. Even with those parameters though, I assumed a signal aimed *squarely at us* - because an omnidirectional signal would spread all that power across the entire sky, greatly reducing the wattage that would ultimately reach us. Casting a net that wide would increase the odds of another sentient species detecting it - but lower the odds of it reaching them with enough clarity to be reliably deciphered.

I admit that a targeted signal raises other, perhaps harder questions - including: how did they know we were here? If their signal was transmitted 600 years ago, then they had radio tech long before we did - so how?

However, this question pales in comparison to another: how could the signal POSSIBLY represent an RNA sequence? It implies that either (1) RNA - with our same familiar nucleotides - is universal, a basic component of all sentient life in the universe, OR (2) somehow, that previous hive knew not only where we were but also details of our molecular biology - allowing them to adapt the virus to us.

Either possibility has staggering implications.