Yes. Male adults are referred to as "Mr." whether they are married or not.
Mr and Mrs its just more formal today in society.
In English, either order is acceptable, but Mr. and Mrs. Jones is the more commonly used order.
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")
Mr. & Mrs. Smith
In the US it is normally Mr. and Mrs.
Dr. and Mrs. or Mr. and Dr.
I think it's because by calling the parents Mr. and Mrs., it shows more respect towards the family
Fru, or hustru (more formal).
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".