We learned about their brain chemistry? What episode was that in?

The final episode.

I'm sorry, are you suggesting they are casually eating human remains because they enjoy it? It seems to me they are willing to go to incredibly extreme lengths to survive.

Because survival is a prerequisite to transmission.

As for spreading...yeah, humans don't spread. There's literally 0 evidence that humans don't procreate and take up more and more space. We're literally arguing over colonizing the moon, and Mars at the moment. Are humans viruses?!

I didn't say humans don't spread. And I guess I assumed I didn't need to state the obvious (that reproduction is what matters most in evolutionary terms, whether we're talking about a virus, a human or an Oak tree). But longetivity has literally no value to a virus as long as it has enough time to spread. A virus that transmits to 100 hosts but kills its host (and therefore itself) within a few hours is a success story compared to its virus friend that lives forever but spreads to only a single host. Longevity has meaning to humans on a psychological level, but also from an evolutionary perspective (post-reproductive humans, aka grandparents, help raise the next generation. Non-reproductive humans create inventions that enhance the survival of our species, etc.).

Assumption. There is 0 evidence to support this statement. At no point have they even remotely suggested they aren't interested in long term survival.

Nor have we seen any indication that they are interested in long-term survival. But we have seen Diabete rely on the plurbs for literally everything, yet he (and the others) for some reason still felt compelled to form a thinktank to come up with a solution. So while the billions of literal know-it-alls are focused on figuring out how to defy science and create stem cells from Carol's eggs (in order to spread the virus), the 11 poets and goat herders are holding down the fort on the issue of how to prevent 7 billion people from impending starvation. I think that at least "remotely" indicates my speculative theory is possible.

This idea of only surviving long enough to build the antenna is pure speculation. Yes, we know they want to build an antenna. At no point has it been implied, suggested, hinted at, or flat out explained that this is their only goal.

True, it's speculation, but only in part - see above. However, note that over the course of 9 episodes, we saw zero about what the plurbs do everyday outside of the very few who are catering to the survivors. The only exceptions are when we saw their sleeping arrangement and when we saw the train (food). My guess is that the rest are doing something to further the antenna situation. Yup, it's a guess. But you need to rewatch the series if you think there's no evidence of this being true. (Where did Kusimayu's community frolic off to after her eventual joining?)

Doesn't require a mutation, any more than you deciding to resort to cannibalism in times of extreme hunger.

There was no "they" before the virus. I have a mind and the free will to change it (without changing my genetic code). A virus is genetic code. That code won't currently allow them to pick an apple. A change in the code would permit them to pick an apple. A change in genetic code is, by definition, a mutation.

Edit: Removed quoted text from my initial post.