I believe she was already divorced from Mr. Brown nor that he was a passenger on the Titanic.
Mr and Mrs
Mr and Mrs is correct or Mrs and Mr is correct
Mr. and Mrs. (ex. "Hello, Mr. and Mrs. Evans" instead of "Hello, Mrs. and Mr. Evans")
Bravo Mr- Strauss - 1943 was released on: USA: 26 February 1943
Mr. & Mrs. Smith
Mr. Andrews had died on the Titanic voyage.
In the US it is normally Mr. and Mrs.
Dr. and Mrs. or Mr. and Dr.
Mr. Lucien Philip Smith and Mrs. Mary Eloise Smith (nee Hughes). They were both from Huntington West Virginia. Mrs. Smith Survived, her husband did not.
I would address it to Mr and Mrs David Keys. For the salutation I would use Dear Mr and Mrs Keys.
Mr. used to be acceptable to address a household by just using the last name, e.g. "Mr. Jones". In modern times, it is typically to use "Mr. and Mrs. Jones" or "Mr. and Mrs. Timothy Jones".