Short answers, pulled from the story.
Mahinda arrived in Sri Lanka during the third century BCE while King Devanampiya Tissa ruled from 307 to 267 BCE. The elder Mahinda was the son of Emperor Ashoka and traveled to convert the king.
The three main subdivisions were the Mahavihara, Abhayagiri Vihara, and Jetavana Vihara. King Mahasena supported the Mahayana-influenced sects between 277 and 304 CE which allowed Abhayagiri to become the largest tradition for centuries.
Weliwita Sri Saranankara Thero restored higher ordination in the late eighteenth century. He invited monks from Thailand to found the modern Siam Nikaya and later the Amarapura Nikaya formed in the first decade of the nineteenth century.
The Freedom Party introduced the Sinhala Only Act which made Sinhala the sole official language in 1956. This decision alienated many minorities including Sri Lankan Tamils who sought regional autonomy.
Over one hundred hermitages housed around six thousand monks according to studies from 1973. Key figures like Kadavadduve Jinavana helped establish associations such as Nissarana Vanaya during this period.