Dear Mr. President is a salutation of a business letter. The salutation goes at the beginning, thus that's how you got Dear.
Traditionally, Mr and Mrs (insert last name). She took his name in marriage.
I would address it to Mr and Mrs David Keys. For the salutation I would use Dear Mr and Mrs Keys.
Mr. Commissioner
Mr. President
Master
Mr. Mayor
Mr. John Jones.
The salutation of a letter addressed to two men named Smith is:Dear Messrs. Smith,The plural of Mr. is Messrs. from the abbreviation of French messieurs (which is the plural of monsieur).
The proper salutation for a psychologist is "Dr." if they hold a doctoral degree (PhD or PsyD) or "Mr." or "Ms." if they do not have a doctoral degree. It is always best to refer to them based on their level of education and preference.
Yes.
Mr President.