Stuart, yes there is truth in what you say. When Garbo decided to leave, she went to Mayer, and it was a mutual agreement that she should leave. He gave her a check to hold her over until after the war, but she refused it saying she didn't earn it. Both, of them, however, did expect her to return sometime later in the future. Garbo was obsessed by her appearance. There is one story of a party guest accidentally walking in on her in the ladies room at some soiree. Garbo was bent over the sink running her fingers over the lines in her face, bemoaning the onset of "maturity." I am concinced that she eventally never returned due to her insecurity about her looks.
Another story is that after the failure of the funding of "The Duchess de Langeais" with the European backers, she retreated permenantly. She has been quoted as saying at the time, "Nobody wants Garbo, anymore," or something to that effect. She had never had to go out and promote herself. She was used to having everything she requested handed to her, and when the backers decided the whole project was too much of a risk, she retreated forever behind her mystery.
Like you said, it was a shame. Funny, I never thought of Garbo as Ilsa. She would have been around 37 to Bergman's 30, which may or may not have made a difference. Bogey and Garbo would have been a SMASH! And, again, I agree with you about the Yellow Rolls Royce. Another role many people have suggested she could have played was My Cousin Rachel, which went to Oliva DeHavilland. But, I wonder if she would have found that role to her liking. She once said, "No mothers, and no murders." That is why she turned down the role in The Paradine Case. I have often wondered if she would have done the role in Gaslight. It would have been great to see her slowly driven mad! After all, she "suffered" like no one else!