In regards to the watcher, I was under the impression that the games follow book canon as backstory right till the start of game (back)story? Is there a book where you could say everything up to it is also canon in the game lore or is the game backstory completely different just inspired by the book?
Yes the games use some backstories from the books/lore, but also this is been written by CDPR (I point to the flashbacks from tw2, those are made by CDPR). Andrzej Sapkowski described it in his own way, and wel he is the creator of them (books). The games continue were the official lore ended, were Yennefer and Geralt died and were left by Ciri on the island of Avalon. Ciri ends up with her love Galahad in our world (king Arthur time).
CDPR made a adaption and created their own stories ( with multiple writers). These stories are not from the original creator, who invented his own characters and stories etc.
The games are a adaption/inspiration of the books and thus are not from the official creator and thus are not canon. Andrzej Sapkowski could write (if he ever wants to do that again) a follow up were he left and then that would be considered canon .