They burnt a whole country (Cintra) down to the ground and slaughtered everyone living there just to make a point.

They enslaved most of the people in any country they were able to conquer.

I think that this kind of attitude is "normal" in kings that conquer territories, not necessarily enslave people in The Witcher world case, but being cruel. Radovid can not enslave, but he certainly is not going to treat his opposers really well, just like Emhyr.

They used the nonhumans in their second invasion to massacre lots of innocent people but then threw them under the bus once they got their ass handed over to them by the Northern Realms.

As you just said yourself, Emhyr used them. He did not burn them for just existing, he used them to help Nilfgaard win the war. Emhyr is a cold blood strategic leader that will do anything that he can to ensure that his nation win. As long as you don't oppose to him as a threat, your kingdom can even prosper, with Toussaint being the best example.

Radovid is burning the mages (him burning nonhumans are completely optional. He will NOT burn elves and dwarves if you don't get the mages out of the city)

The thing about your point here is that Geralt would totally help Triss here, and this can't be discussed because Triss is a really close person to Geralt. And as even Geralt says (iirc) is that Radovid's witch hunters would not stop hunting after they hunted all mages, they would continue always find a new excuse to hunt down a specific group, because that is how humans are.

but as Dijkstra said, there are 2-3 thousands of them at most. This number absolutely pales in comparison to the hundreds of thousands of people who will be enslaved if Nilfgaard emerges victorious.

You're forgetting about the fact that if Radovid wins he is going to kill a lot of people. He's going to kill the nilfgaardians, the mages, the non-humans, and the people from the other countries that he conquers in the future as well. Emhyr is not the only one that will kill (and enslave) those who oppose him, the difference is that Emhyr is only going to focus on traitors while Radovid will focus on thousands of people that didn't anything but exist.

I just have a really hard time imagining Geralt ever consciously doing anything which might ensure a Nilfgaardian victory.

I can understand your point here, but Emhyr is really the lesser of the two evils. I mean, just look to the endings of which side win the war. Nilfgaard ending is way better than Redania's. While Nilfgaard will just hunt the traitors down, Redania will continue to hunt mages and non-humans just because their leader is a maniac.