they died by a heart attack
no
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")
On an envelope as part of the address: Familie When referring to a couple: die ..... e.g. that's Mr and Mrs Bauer - das sind die Bauers das Ehepaar e.g. that's Mr and Mrs Bauer - das ist das Ehepaar Bauer
Mrs. Andrew Whitehead died in 1920.
Mr. & Mrs. Smith
In the US it is normally Mr. and Mrs.
No, both Mr and Mrs Weasley survive.
Not by Dragon.
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.