What is the ruling on laughing at the whispers of Satan, and is the one who does so sinful? And what is the ruling on laughing at gloating, backbiting, or self-talk, and is the one who does so sinful?

There is no sin in laughing at self-talk and waswas (whispers of Shaytan/obsessive thoughts), for Allah has forgiven my Ummah for whatever thoughts they have, as long as they do not act upon them or speak them. This laughter might be out of astonishment at the waswas and not an acceptance of them. You must strive against these waswas and get rid of them.