Some might be willing to bite the bullet at this point, trusting some strongly held ethical principle of theirs (e.g. A, B, C, D, or E above), to the conclusion of excluding humans who lack certain cognitive capacities from moral concern. One could point out that people's empathy and indirect considerations about human rights, societal stability and so on, will ensure that this "loophole" in such an ethical view almost certainly remains without consequences for beings with human DNA. It is a convenient Schelling point after all to care about all humans (or at least all humans outside their mother's womb).

This is pretty much my view. You dismiss it as unacceptable and absurd, but I would be interested in more detail on why you think that.

a society in which some babies were (factory-)farmed would be totally fine as long as the people are okay with it

This definitely hits the absurdity heuristic, but I think it is fine. The problem with the Babyeaters in Three Worlds Collide is not that they eat their young but that "the alien children, though their bodies were tiny, had full-sized brains. They could talk. They protested as they were eaten, in the flickering internal lights that the aliens used to communicate."

If I was told that some evil scientist would first operate on my brain to (temporarily) lower my IQ and cognitive abilities, and then torture me afterwards, it is not like I will be less afraid of the torture or care less about averting it!

I would. Similarly if I were going to undergo torture I would be very glad if my capacity to form long term memories would be temporarily disabled.

(Speciesism has always seemed like a straw-man to me. How could someone with a reductionist worldview think that species classification matters morally? The "why species membership really is an absurd criterion" section is completely reasonable, reasonable enough that I have trouble seeing non-religious arguments against.)