Formal titles.
I would address it to Mr and Mrs David Keys. For the salutation I would use Dear Mr and Mrs Keys.
Yes. It would be appropriate to say "Mr. and Mrs. Smith" for example.
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".
Mr Bean's landlady in the animated series is Mrs Wickett.
Mr and Mrs
Mrs McGrath
Honorific
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")
Yes.
These are called honorifics or terms of address.
Mr. & Mrs. Smith