One of the major reasons WS don’t tell the truth is that they are afraid of the consequences.
Think about it – think back to when you were a kid. You probably did something at some point that you found hard to confess to your parents. They had to pull each and every part of it out of you. It’s the same with your WW; she’s afraid, ashamed or embarrassed to tell you the truth.
The only way IMHO to get the truth out is to make not telling the truth too expensive.
Sort of make the truth the lesser of two evils for her.
What you can do is compile a list of key issues you need answered. The answers to those will probably generate a new list with further questions, but start off with the general major issue list. Then make it clear to your wife that there is not the slightest chance in hell that you two can reconcile and/or the marriage be saved UNLESS it’s from a base of total truth.
You have to be firm on this: Without the truth there really isn’t any need or requirement to work on the marriage. There really isn’t a realistic option to reconcile. So why spend any effort on a doomed project. So if she wants to divorce she keeps her mouth shut – if she wants to work on the marriage then the price for the possibility of that option is speaking frankly about the affair.
At the same time you need to understand and possibly get some points across to your wife:
You have to make it clear to her that whatever she says is not likely to make things worse. That discovering she had sex with him 5 times rather than 4, that she did 29 positions rather than 6 or whatever won’t really make things much worse. Sort of like being stuck by a needle won’t really make much of a difference if you are being hacked by a sword at the same time.
Then you have to realize the questions need to have a purpose. For example – I have seen more cases than I care for where BH have asked about OM penis-size. The answer is always a lose-lose item. If WW says it’s smaller the BH doesn’t believe her. If she says it was bigger… well that tends to hit hard. So when you think of the issues you need answered then THINK: Does the question serve a purpose, is it answerable and is necessary for the marriage.
You can also try to create a situation that is conductive to your WW answering. For example: You sit back-to-back so she doesn’t have to look at you. You do this in a controlled environment where there are no kids, phone or disturbances. You give her a list and she answers without interruption…
Like I say - the answers will very likely generate new questions so you definitely need more than one session.
If you get to the stage where she opens up then plan carefully: Take an hour to answer questions, then go your separate ways for an hour to run, walk, the gym… Then spend time together NOT talking about the affair.
Finally – do not hesitate to require a polygraph. Don’t be afraid of leveling the “if you fail the polygraph based on what you have already told me this marriage is over” threat. Remember: YOU NEED the truth to move on.