Is saying "What do you find so amazing about it?" in response to the question "What is the story behind this beard?" considered mockery of religion, especially given that the questioner suffers from whisperings (waswasah) regarding this matter?

You must be careful not to dwell on obsessive thoughts, and what happened is not mockery but rather the effects of these obsessions. Your brother's question about the beard in this manner is inappropriate, especially if he knows that it is among the emphasized Sunnahs. He must venerate the rituals of Allah, and if he is unaware that it is a Sunnah, then you must teach him that.