Well, the point is the game is still treating all of Sapkowski's writing as canon. You'll get more out knowing Emhyr from the novels than you will out of knowing the entirety of what happened in W2, basically.
As to Geralt that's up for debate...
Different teams/studios working concurrently on different projects. And one of those studios is also remaking the 1st game, in UE. There's more projects in the pipe for CDPR, also a new IP. This expansion is sort of the answer to development...