It took me a few minutes to bring my jaw off my desk....wow.
I am going to point out something that is strickly my own opinon and not meant to be judgemental. But if I were your wife and found out that you were having these discussions without my knowledge with the neighbour, I would be just as hurt, angry and upset as if you had actually cheated on me.
Swinging is based on trust, communication and love. If you don't have those things it just isn't going to work. If she is saying no because she is feeling insecure, you aren't doing anything to alay her fears and make her feel confident that you love her and only her, you are doing quite the opposite.
I think the best advice for you right now is to put yourself in your wife's shoes and try to understand where she is coming from, and then you maybe you may have a chance at getting her to talk about it. Swinging is not for everyone and maybe it's not for her (and that doesn't make her a bad person either) , I guess you have to really decide which is more important,swinging or your marriage.
Best of luck