What is the correct source for learning Islam, what should one do when scholars differ on an issue, what is the saved sect today, what should a person do if they suspect they have committed an act of disbelief, and how can they be sure they are still Muslim?

The sources of religion are the Quran and the Sunnah (definition). One who is able to know the evidence should follow it, and one who is not able should imitate someone whose knowledge and piety they trust. If the muftis differ, one should follow the opinion of the most knowledgeable. As for the dissolute person (fasiq), they do not commit an act of disbelief (kufr) unless they deem a sinful act permissible or mock religious rituals. A Muslim remains steadfast in their Islam with certainty and does not abandon it due to doubt. They do not commit an act of disbelief (kufr) except through an action, statement, or belief that the Quran and Sunnah have indicated is major disbelief (kufr akbar). What the questioner mentioned indicates whispering (waswasah), and they should repent from the forbidden acts and not give in to these whispers.