If you have no strong ideas about the blameworthiness of the three characters, you may wish to write the prequel before you assign the percentages. From a literary point of view, allocating a certain amount of blame to each character is the least interesting thing about the idea of writing a prequel. It is also the easiest.
Although you can include any characters you like, unless you have a clear alternative idea, you will want, at a bare minimum, some dialogue between John and Elizabeth and some between John and Abigail. Fortunately, you have good models for this. Look at Miller's dialogue between John and Abigail in act 1 and between John and Elizabeth in act 2. The first thing you will notice is how strained their conversation is in both cases. Was this always a feature of both relationships? If you think it was, you might begin by showing John and Elizabeth failing to communicate and John seeking solace from Abigail.
The dynamics of the way their affair begins will require skillful handling. John is older than Abigail, and he is the married party. These facts suggest that he is the more responsible of the two. However, Miller depicts Abigail as being extremely skilled in manipulating others. You might show how Abigail uses the difference in their positions to make John feel guilty and do as she wishes. Remember that you can use stage directions as well as dialogue to accomplish this.