Are isolated (Ahad) Hadith to be used as a basis for legislation, and why did the Prophet (peace be upon him) not publicize legislations so that they would become متواترة (continuously recurrent/mass-transmitted)? And have all of his sermons reached us?

Ahad (single-narrator) hadith (definition) is a proof that must be acted upon, and by it, legal rulings are established. The Prophet's (peace be upon him) clarification of religious matters was not dependent on the availability of a mutawatir (mass-transmitted) number of narrators. The same was true for those whom the Prophet sent to teach people their religion. Gathering a mutawatir number for every ruling is a difficult matter, especially given the Prophet's guidance against prolonging sermons; Jabir ibn Samura said, "His prayer was moderate and his sermon was moderate," and Abu Wail said, "Indeed, a man's long prayer and short sermon are a sign of his understanding." The Prophet made his sermons for admonition, the truths of faith, and comprehensive matters, and "he would deliver sermons at all times according to the needs and interests of the addressed." A considerable number of his sermons (peace be upon him) have been narrated.