Why she rejected Joe, My thoughts
If the woman whom loved her would not get Karen then the same for the guy whom loved her. Her excuse for not marrying Joe made no sense to me. He was kind and really loved her just as Martha did. This is why her rejecting him made no real connection to viewers. There breakup scene was hard to write because when it is forced to only try and prove no one will get Karen it rang hollow. It really was not needed to show she will not accept him because she lost her best friend whom loved her also.
The same thing probably would have happened if Joe committed suicide. She would not have left with Martha either. I Don't understand why some writers like for people to be alone after losing someone.