I have learned a great deal being here on and off for a number of years. First of all, IMO, your wife's therapist is as wrong as they get when she told your wife to lie. Lying should never ever be recommended by a therapist unless there is a real threat of violence with the truth. Your wife should never talk to this therapist again.
If this therapist had told your wife to be honest, your DD would have been 2 years ago and you and your wife could have been over this part. For a BS, DDay is just as if it just happened, 2 years ago means nothing for a BS, but to a WS, it was actually 2 years ago.
In order for you wife to be totally honest with you about what really happened, she has to be totally comfortable telling you. And that means no threats of divorce, no screaming anger from you, and no threats at all about telling anyone.
Otherwise, what real motivation would she have in telling you any truths, if you are going to hurt her in any way or divorce her. It sounds odd, that SHE has to be comfortable, but that is how it is if the BS wants the entire truth. And most, not all, but 99% of the BS I have ever read a post from not only needs the entire truth, but has to have it in order to move on and heal from this all.
Her depression and her childhood could be an actual reason for her affair. Many WW do have something bad in their childhood like low self esteem from many reasons and or being sexually molested at a very young age that can cause them to have some kind of affair later in life.
Those feelings of her low self esteem and other childhood issues should be discussed in order for her to never need an affair again.
As for whether or not she was in love, who knows. Puppy love, unreal love, fake love, pretend love are all common in most affairs. Once the affair is over, many times, that pretend love for the OP turns into real disgust.
In order to get over this, you need to calm down and stop making any threats about telling her family. You can ask this other guy some questions if you want to, you might even get answers, but do not threaten him with harm or you could be in serious trouble. That almost happened to me.
You need to explain to your wife why you need the truth, BUT make damn sure you do need the entire truth and that you can handle the truth. You are already re-reading those emails in your head and getting sick and angry. That is why I write, make sure you can handle the truth.
When it comes to the truth, it is going to make you angry and depressed at the same time. But with time you will actually get used to the truth and those feelings will fade away. Which to me, was always better than never knowing the full truth and always wondering.
I am curious about one thing. You wrote that this other guy was married at the time. Is he divorced now? And if so, I wonder what the reason was for the divorce. Did his wife find out and or did this guy have other affairs as well.