Is Triss a pale redhead in the books?
Not exactly.
She has chestnut brown hair, wearing them long and unbound, blue eyes not green, not sure how pale she is, but yeah description points to white, and she always dresses without cleavage in books after battle of Sodden due to scars.
"That time" is naturally going to be vague, as its not an alternate history.
It's not history at all. If any history is distant future of current humanity after cataclysm destroyed Earth - but it's not certain.
You've got late middle ages tech with the longswords etc, and the Nilfgaardians who seem clearly based on the Black Army of Hungary, which came later. Somewhere in the 13-1400's would be my guess.
You also have academics on unviersities using modern terms and for instance modern patterns of naming animals which was invented way after medieval period.
Nilfgaardians are based on Netherlands in terms on language, and mostly on Holy Roman Empire in times of it's strong cohesion with notes of Roman Empire proper and also Third Reich (especially early in books they are quite obviously nazi proxies)
During that period you've got quite a lot of war with the Ottomans right across the border from Hungary. Before that, the Mongols had invaded Hungary, Poland, and others. Before that, Crusaders and/or Byzantines controlled large chunks of the Medditeranean coast on the Africa and Middle East sides, with established trade routes. And of course well before all of that the Romans were invading or being invaded by Carthage.
So I have to disagree. Certainly they were there, at minimum as invading armies. I don't think its that far fetched that some hirelings might be brought back from those places once controlled by the Euros, and I would imagine there would be at least some mixed race individuals as a natural result of invasion and occupation. There are still descendants of Ghengis Khan in Eastern Europe.
I mean I doubt Middle Easterners and North Africans who are all variants of mediterranid "subrace" would be considered in Byzantium as very different race. Different cultures, yes, but half-Greek, half-Syrian would not look outside of norm.
With East Asians it's true both ancient Uralic invasion and Mongol one left few % of Asian DNA among Eastern Europeans.
Nevertheless that's bit different to have a guy with 1/4 of Mongol blood (and Mongol armies where only part EA - partially they were white people of Western Asia) and just random ethnicities around. But anyway such technicalities do not matter for Witcher world much.
How would SJWs react if Wakanda suddenly had a bunch of Asians and Latinos running around? I'm not even talking about adding those evil honkies, but adding fellow non-whites into the Afro-fantasy concept.
Just because SJWs are dumbass, I'm not gonna go and mirror them. Margaret of Anjou as black woman was retarded, Anna Boleyn - probably also unless they go really postmodern and anachronistic fully with this new show, I'd have a problem with black Rohirrim or Hobbits. With Witcher world I'm bit meh, but I don't care much - author himself claim he does not care about worldbuilding - world is just a stage to be shifted for his stories, it does not matter on it's own for him.
Asians in Wakanda would be also stupid, but some Asians in some fictional vaguely defined world based on Zulu legends - YES DO IT.
My greatest desire is to see cyberpunk thriller based on Zulu mythology, with exclusively Korean cast, written in Navajo, filmed entirely in Estonia by Persian director and Brazilian producers. Do it - and I can die happy.