Is a smile considered mockery when a colleague jokingly says, "Why do we make things difficult, and so on," bearing in mind that the Prophet (peace be upon him) always chose the easier of two options? And should one fear falling into disbelief?

What you have mentioned is merely a whisper (waswasah), so do not pay attention to it. Rather, turn away from it, for it is one of Satan's tricks and plots against you. He intends thereby to spoil your life and corrupt your religion and your worldly affairs. There is nothing in what you have mentioned that relates to disbelief, neither closely nor remotely.