What is not understood about why Nick Arnstein and Fanny Brice split up?
Not clear to whom? He was overshadowed by his wife and it affected his ability to win at poker. She was making more money and he decided to do something shady to earn money.
No mystery there at all.
Why would Columbia give Streisand rights to alter the film's ending? As it was her first film, what concessions could they have made?