What is the ruling on excessive obsessive doubts and skepticism regarding Sharia rulings and matters of creed, and is this considered among the causes of apostasy?

You have done well in defending yourself against those whispers (waswas), and continue to turn away from them while seeking Allah's help. Thoughts and inner monologues of the self are not held against a person as long as they do not settle in the heart, based on the words of the Almighty: "But He will hold you accountable for what your hearts have earned," and the words of the Prophet, peace and blessings be upon him: "Indeed, Allah has overlooked for my Ummah that which their hearts whisper, as long as they do not act upon it or speak of it." Scholars have differed regarding the ruling on congregational prayer.