What happens if you fail a quest Witcher 3?

If the quest is needed directly for an achievement then you can’t get it, but most side quests aren’t. You’ll still get the one for completing the game even if you don’t complete a single side quest.

Should you kill Radovid?

If you choose not to help kill Radovid, or you side with Sigismun Djikstra, then the empire will lose the war and be overthrown. If instead you do help kill of Radovid and you side with Vernon Roche in Reason of State then Nilfgaard will conquer the north.

Is Isle of Mists the end?

Isle of Mists isn’t really “end game”. It is the end of Act 1. (You should know what your task is, and once you have seen what awaits you on the Isle, have some idea how the end game plays out).

How do I get everyone to kaer morhen?

That’s when you are supposed to recruit people for your upcoming battle in Kaer Morhen. You must play a series of lengthy side quests and subplots to recruit everyone, which is going to take several hours. Keira Metz and Triss can be recruited before reaching this point in the game and are the easiest to miss.