Does apostasy occur due to ignorance or forgetfulness by saying: "Insha'Allah (God willing) or for sure," or perhaps smiling without intending to mock the religion, or upon hearing the phrase "Jesus lying down"?

The cure for obsessions (waswas) is to disregard them, ignore them, and pay no attention to them. No matter what form they take, ignore them. Obsessive thoughts about leaving the fold of Islam do not harm you as long as you dislike and are repulsed by them. Your being a Muslim is not affected by these obsessions. There is nothing in what you mentioned that necessitates leaving the fold of Islam. Therefore, be at ease, ignore these obsessions, and do not give them any importance.