The level we can actually suspend our disbelief against our own experiences seems to be the issue here. Your points are entirely valid, from a scientific standpoint. But we're not talking about science! Rather we are talking about a fantasy setting. Some elements must be fantastic.
No, no, sure, on both counts. It's a fact, at this point, that I take it too seriously.
I find it very interesting that elements like sorcery are accepted, but not Kellhus' other abilities. Sorcery has no analogue, so it is easy to accept it as fantastic. Elements that do have an analogue, and R. Scott Bakker's (dis?)ability to express them or not, are harder to accept when we do have our own challenges to compare it to.
I'm fairly certain this is exactly the issue here, so it's unlikely we have a point of contention.
I'll add a bit of an expansion of my views on the matter to hopefully bring more clarity. As I said before, the most fantastic thing for me is the way Kellhus manipulates people. In essence, all stories are about interpersonal interactions. The worlds, the cultures, even many completely physical aspects recounted are not as important as what people do in regards to each other. Our stories are about people impacting people, this is what we look for first and foremost. This is also why upsetting these exact dynamics (in the sense of significantly contradicting personal experiences and often observed scenarios) is so grating. From my perspective, at least.
Kellhus has intimate knowledge of the causality of the emotions involved. Yes, if you are looking at somebody you have no knowledge of, there can be a million reasons. Kellhus is with them to see the stimuli. He can see the cause and effect that lead to the emotional response which is far, far, less than millions.
Alas, it's still millions. Every reaction is rooted in the experiences of a person. Those experiences are unique, and they produce unique reactions. The stimuli may be observed, as they often are, but discerning thoughts and clear meanings behind emotions those stimuli invoke, that's exceptionally hard. In some cases it might be hard even after getting completely truthful clarification from the person in question.
Yes, you might, with time and exposure, gather enough information to start making correct deductions, but the amount of information required would take years for a single person to accumulate, which is, incidentally, a realistic timeframe for learning a language.
The Dunyain have a different biology, though! I do accept that if these biological limitations have been bred for, perhaps some can be overcome. If Kellhus does have a memory that does not exist in our own biology, does the rest of what he does still fail?
I feel that a species that different is just so beyond human ken that there is no hope of interaction. If their biological framework is so different and advanced, then the Dunyain (or other similarly advanced beings) have nothing to gain on human level, there is no common ground.
Having this kind of memory with the ability to autonomously and rationally function changes that much in my eyes.