The envelope would be addressed as:
The Honorable John Smith
United States Senate
Washington D. C. 20515
In letter:
Dear Senator, or Dear Senator Smith
Senator Doe,
The proper salutation for a retired state senator is typically "Senator (Last Name)," even after they have left office.
To whom it may concern
Rabbi and Mrs.
Dear Counselors:
A proper salutation for greeting the president of a business is "Dear Mr. President". To conclude a letter, the proper word is "Sincerely" and a the name of the writer.
Dear Mrs Ambassador..
vmrs
"Dear" is what people use it nowadays.
yes
The proper salutation for a letter to someone with an MDiv degree would be "Reverend" if they are ordained as a minister. If they are not ordained, you can address them as "Mr." or "Ms." followed by their last name.
Governor. If it was a previous governor you would simply say former Governor.