Honestly I think you are caving into pressure and in the end you are going to be unhappy and it is going to cause stress on your marriage.
Swinging only works if everyone is happy, what you are doing is the equivalent of taking one for the team to keep your wife happy. Your feelings are legitimate and you have every right to be apprehensive. How would your wife feel if the roles were reversed and she had to sit at home while you were out playing with some other woman who wanted a realtionship with just you. I have a feeling it would not go over well.
It sounds to me like your wife is being a little selfish and childish...you say she begged and pleaded with you until you said yes...then she was peeved because you had to come home and go to bed...sounds like she needs to stop thinking about her needs and remember that you are supposed to be the most important person in her life...swinging is just an activity.
Ask yourself what you are truly comfortable with and then go from there. I would be a little nervous about her playing with someone she works with every day....is this woman going to respect your marriage....Are you completey comfortable with her, has she given you the chance to get to know her, after all you are letting her share something very intimate and personal with your wife, don't you think you deserve that much?