Yes. It would be appropriate to say "Mr. and Mrs. Smith" for example.
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".
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")
Yes.
Formal titles.
Mr. & Mrs. Smith
In the US it is normally Mr. and Mrs.
No. A person can use "Mr" or "Mrs" or any variation of these, such as "Miss" in the first person. For example, "Hello, I'm Mrs Jones."
Yes. Period. Yes, you do.
Yes