I know this has been asked before, but I am asking again

2 questions
1) For a formal wedding did you address them as formal as the wedding invitations so Mr. and Mrs. Joe Smith or less formal so John and Mary Smith. THe wedding is very formal, but the STDs are a little more fun and less formal.
2) Couples that are unmarried. We have a lot of these so if they live together address to the person we know best first and then the second person.
Now what about for those that do not live together, some of the significant others I know fairly well and don't want to offend. Also, some of my friends that I do not know their significant others gave me their names (they do not live together) should I just address as if they were living together? The only real concern is if they break up, but they would know they weren't invited anymore right.....