Yes.
Period. Yes, you do.
Yes, i.e. Mr. , Mrs.
No, the abbreviation "Mrs" does not require a period.
You put a period after all of those in the states, however, the British system requires no period after such abbreviations, Mr Mrs Ms .
Yes, Mrs. has a period because it is an abbreviation and mrs is not a word. The same holds true for Mr., Dr., and Ms.
You would say the male title first, but you need and in there. It should be Mr. and Mrs. John, and Mr. needs a period.
No, just use one space.
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 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")