Short answers, pulled from the story.
Lennon and McCartney first met on the 6th of July 1957 at a church fête, where McCartney impressed Lennon with his guitar playing. The partnership was formalized in legal terms on the 1st of October 1962, when Lennon, McCartney, and Brian Epstein signed a publishing agreement with Ardmore and Beechwood.
Between the 5th of October 1962 and the 8th of May 1970, the Lennon-McCartney partnership published approximately 180 jointly credited songs. The Beatles sold more than 600 million records worldwide.
"Yesterday" is generally attributed primarily to McCartney. The credit order dispute around the song became a focal point of a later dispute between McCartney and Yoko Ono in the late 1990s and early 2000s, which McCartney eventually conceded in 2003.
The first Beatles single used "Lennon-McCartney", but the next three releases reversed it to "McCartney-Lennon". An April 1963 band meeting settled on Lennon first, and that order was maintained on all subsequent official releases. A later dispute in the early 2000s over McCartney's use of reversed credits on live albums ended in 2003 when McCartney stated he was content with the original order.
No. Lennon and McCartney privately discussed including Harrison when formalizing the partnership in 1962 but rejected the idea. Harrison later said the arrangement made him feel like an observer, though McCartney acknowledged in later years that Harrison made vital uncredited contributions to Lennon-McCartney songs, including the guitar riff in "And I Love Her".
According to Guinness World Records, "Yesterday" has been recorded by more musicians than any other song in history.