Does the questioner become a disbeliever in the following cases: due to his whispers that led him to fear creatures more than Allah, and abandoning prayer, and intentionally delaying it, or slandering scholars, or praying the Sunnah for fear of being described as a hypocrite? And is his feeling of extreme remorse after knowing that slandering scholars is disbelief sufficient for him to become a Muslim?

You are a believer, and the whisperings you experience are clear signs of faith and a great trial. You must not abandon prayer due to these whisperings; rather, it is obligatory to resist them. Respecting scholars is a duty, and laughing at them is not apostasy. And whoever performs good deeds out of fear of people's كلام (speech/opinion) does not become a disbeliever by that, but it is a form of forbidden ostentation (riya') and is considered minor shirk.