I'm almost 100% sure that they refer to people as "Mr. Henderson" or "Mrs. Jacobson" all the time. They might refer to someone by their first name if they are introducing them to the court for the first time.
In all there are 12 ! ways. If Mr and Mrs S are treated as a single unit, then there are 11! ways of arranging the 10 other people with Mr and Mrs S together. But these can be Mr-Mrs or Mrs-Mr so, in fact, there are 2*11! ways in which Mr and Mrs S are together. So the number of ways in which they are NOT together is 12! - 2*11! = 399,168,000
Mr and Mrs
They are two people - two is plural, so it should be "Where are Mr and Mrs Jones?"
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")
Mr. & Mrs. Smith
Lawyers generally possess a Juris Doctor (J.D.) degree. You can address them as Mr., Mrs., Miss, Ms., or Dr.
In the US it is normally Mr. and Mrs.
A prefix on an application for employment may refer to Mr. Miss, Ms. or Mrs. This is a title chosen by the applicant.
Dr. and Mrs. or Mr. and Dr.
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".