Lulu's decision to Geass Schneizel and keep him alive (instead of having him killed at some point) doesn't make sense. He already knows that Geass can be nullified with science (as shown by Jeremiah) and he is about to see his sister break the Geass Charles put on her. Why just not off Schneizel in one way or another if there is a risk of him ever breaking the command? He's too dangerous to stay alive.