Mrs, Mrs & Mrs
"Dr and Mrs Smith" is the correct salutation. (With Smith replaced with the correct surname)
On the Envelope His/Her Excellency Mr./Ms./Dr./Rev. [name] Ambassador (or High Commissioner) of [country in full] In letter Dear Mr/Mrs Ambassador
I would address it to Mr and Mrs David Keys. For the salutation I would use Dear Mr and Mrs Keys.
Rev. when in written form. Reverend in spoken form.
Mrs. or Ms.
The Reverand and Mrs
Yes.
Rabbi and Mrs.
Dr. and Mr. Smith.
Dear Mrs. Obama,
Dear Mrs Ambassador..