Is what I mentioned—listening to and relaying jokes that mock religious rites or describe prophets with inappropriate attributes—considered disbelief (kufr)? What is the ruling on laughing at them? How can one deal with doubt about committing an act of disbelief in the past? Does the principle "certainty is not removed by doubt" apply in this case? And is it true that repentance is sufficient for someone who doubts having committed an act of disbelief in the past, without needing to utter the Two Testimonies with the intention of re-entering Islam?

It appears the questioner is suffering from waswas (definition) (whispers/obsessive thoughts), and the advice is to turn away from these waswas and ignore them. Much of what was mentioned does not lead to apostasy; rather, it is something that should not be circulated. Even if acts of disbelief were to occur, general repentance is sufficient to wipe away past sins, with the caveat that the condition of uttering the Two Testimonies for one who repents from disbelief is not incorrect. If one doubts whether a specific act occurred before or after puberty, the default assumption is that it occurred before reaching puberty.