Mrs Smith, unless she has specifically changed her surname back to her maiden name
"Dr and Mrs Smith" is the correct salutation. (With Smith replaced with the correct surname)
Dr. and Mr. Smith.
Dr. Smith and Mr. Smith separately..
Dr. and Mrs. Smith
The Honorable and Mrs. (first name with middle initial, if any) Smith
Yes, it is proper to address a widow by her last married name.
The Honorable Judge Smith and Mr. Smith for the envelope and Judge Smith and Mr Smith for the letter.
Mrs, Mrs & Mrs
The letter salutation is written as: Dear Doctor and Judge Smith. The envelope is written as: Doctor Adam Smith and the Honorable Eve Smith.
Dear Judge Smith: See http://www.factmonster.com/ipka/A0771340.html for more info.
Dear Dr. & Mrs. Smith
Yes.