Were all the hypocrites in the era of the Messenger and the Companions exposed, or did some of them remain unknown? And is everyone whom Ahl al-Sunnah wal-Jama'ah are pleased with definitely a Companion? And did the hypocrites say "The Messenger of Allah, peace be upon him, narrated to us" with fabricated hadiths to the general public? And how can one distinguish between a Companion narrator and a hypocrite narrator?

Yes, the Hadith scholars identified the hypocrites through several indicators, among them: the Quranic verses that described them, the commands to wage jihad against them and to beware of them, Allah’s oaths to His Prophet that he would come to know them, the presence of Hudhayfah (may Allah be pleased with him) who knew them, and the consensus of the Companions on knowing them. Ahl al-Sunnah invoke Allah's pleasure upon the Companions and others, and the Sunnah of the Prophet was protected from falsehood during the Prophet's time (peace be upon him). The two Sheikhs (Bukhari and Muslim) exercised caution in accepting Hadith. It is possible to differentiate between the Companions and the hypocrites through books that focused on narrators and their characteristics, as well as books of al-jarh wa al-ta'dil (discrediting and attesting to the reliability of narrators).