Short answers, pulled from the story.
Benedikt Schack (1758-1826) was an Austrian tenor and composer of the Classical era who was a close personal friend of Mozart. He is best known as the first performer of the role of Tamino in the premiere of Mozart's opera The Magic Flute in 1791.
According to an account first published in an anonymous obituary of Schack in 1827, Schack participated in a rehearsal of Mozart's Requiem on the last day of Mozart's life, the 5th of December 1791. The account describes Mozart weeping during the Lacrimosa and dying eleven hours later. The story appeared 36 years after the event.
Yes. A story published in the Baierisches Musik-Lexikon in Munich in 1811 states that Mozart would sit at Schack's writing desk while Schack dressed and compose passages in his operas. Mozart also wrote eight variations (K. 613) on an aria from Schack's Singspiel Der dumme Gärtner, and contributed a duet (K. 625/592a) to their collaborative opera Der Stein der Weisen.
A record from 1815 indicates that Schack sang the role of Tamino a total of 116 times.
Benedikt Schack was born on the 7th of February 1758 in Mirotice, Bohemia, then part of the Habsburg monarchy. He sang as a cathedral chorister in Prague from 1773 and later moved to Vienna in 1775 to study medicine, philosophy, and singing.
Constanze Mozart, who remarried in 1809 and became Constanze Nissen, wrote to Schack near the end of his life seeking his memories of Mozart for her second husband's biography. She described Schack as the person to whom Mozart was most devoted. Schack died in December 1826 before he could reply.