Short answers, pulled from the story.
The song was inspired by Melanie Coe, a 17-year-old runaway from Stamford Hill, whose story Paul McCartney read in the Daily Mail. Coe later confirmed the song was largely accurate to her experience, with only minor inaccuracies: her boyfriend was a croupier rather than a car dealer, and she left in the afternoon rather than the morning.
The string arrangement was written by Mike Leander, not the Beatles' usual producer George Martin. George Martin was unavailable when McCartney needed the score done quickly, so McCartney hired Leander, making it the first Beatles song arranged by someone other than Martin. Martin still produced and conducted the string session on the 17th of March 1967.
Sheila Bromberg played harp on the track. She is identified as the second female musician to appear on a Beatles record, after cellist Joy Hall, who performed on Strawberry Fields Forever.
Yes. In 1963, McCartney chose Coe as the prize winner in a dancing contest on the ITV programme Ready Steady Go!, four years before he wrote a song based on her story without realizing the connection.
The stereo version, finalized on the 17th of April 1967, runs at a slower speed than the mono mix completed on the 20th of March 1967, which makes it a semitone lower in pitch. A 2007 Mojo magazine article revealed that the mono mix had been deliberately sped up to make McCartney sound younger.
Lennon and McCartney received the 1967 Ivor Novello award for Best Song Musically and Lyrically as the credited composers of She's Leaving Home.