Perhaps they never saw the list for themselves. They may have asked a crewman if there was a Rose DeWitt Bukater on the list, and they would have been told no. I would say that after that they would have accepted her death. Just my thoughts.
You're right though; it would have been pretty easy for them to have found out. Had she not given her actual first name to the crewman who asked, then I could see her survival remaining a mystery.
Unless Ruth died before Rose became an actress or famous. It's presumed by the fact that Rose went ninety years believed to have died on the Titanic (As Bodine said, "Rose Dewitt Bukater died on the Titanic when she was seventeen...") that Ruth might have died beforehand. Rose became an actress in the 1920s, but may have reached her peak after Cal committed suicide in 1929.
Still, you'd think the researchers would have compared older pics of Rose Dewitt Bukater and the later actress (Rose Dawson/Rose Calvert) and put two and two together. After all, researchers are sharp and thorough, right?
All of these are plausible explanations, but the truth is it's hard to come up with an infallible aftermath when the story itself never took place. It is a lot of fun to try though.
I think if Rose had become a household name as an actress, her past would have been uncovered eventually and spread worldwide by the media. That's why I doubt she ever became too terribly famous. I just see her as being an actress with steady work, but no starring roles. But that's just my vision of course.
Yes, those are good points, Brandon. Being an actress in the '20s was not necessarily like today, where their faces are all over magazine covers and TV. It's certainly quite possible she was not a very well known actress.