If your wife has mixed feelings about non-monogamous sex to begin with,
exploring this situation further will do nothing to improve them!! TRUST me! This is where that 'gut instinct' comes in. Something didn't smell right about this woman's intentions? Then listen to that hunch! From what you've said already, there are nothing but red flags everywhere.
1) Your wife doesn't have any interest in swinging, per se.
2) This woman is almost like part of your family (meaning anything goes wrong, and you're looking at some serious upheaval, not just for you, but your kids too.)
3) There were copious amounts of alcohol doing the talking that evening.
4) B is very young. Sorry to any 20-somethings out there, but we've found that folks under 30 generally have less life experience with which to make decisions (Please don't be offended; it's just a fact of life). This being the case, she may not be able to recognize all the factors involved.
5) She approached you while your wife was away and stated (under some assumption of confidentiality) that she was in fact more interested in you than your wife (HUGE red flag).
6) Your wife still doesn't really want to swing.
7) Your wife is interested ONLY in exploring her bi-curiosity (nothing wrong with that), and B is interested MOSTLY in sex with you.
I haven't read all your previous posts yet (I will), but I do recall a post where your wife stated that as long as she was your wife, there would be no swinging. This sex between you and B is NOT something, I gather, your wife is interested in exploring. So. That being the case, this should be a no-go right at that. There are still plenty of other reasons, as stated above, why you shouldn't even be considering this.
I don't think this indicates a problem between you and your wife, and there's no need for attorneys. Just that some boundaries you've both set forth are being tested right now. Do you both want to stick to those boundaries, or have your comfort levels changed? That's something only you and Mrs. Coupleinidaho can hash out together. The problem I'm seeing is that you're letting your libidos influence your decision-making process. It happens to everyone, because it's unbelievably easy to do. The sex drive can be pretty persuasive that way.
But that doesn't take away from the fact that you do need to make these important decisions objectively.
So, my advice would be this: 1) Tell B in no uncertain terms that neither you nor your wife will, under any circumstances, play with her. You value her too much as a trusted employee and family friend. 2) If you're going to have a drink with B again, make sure that at least one of you (you or your wife) stays sober enough to keep things between the lines. Somebody needs to keep their wits about them. Your marriage may depend on this. 3) Fantasize all you like about B, and both of you can share those fantasies
with each other. Do not share your fantasies with B. A similar situation happened between Mr. intuition and I regarding a friend of mine who had expressed some bi-curiosity. It sure heated up our sheets, but because she was a good friend of mine, we chose not to pursue something that could end very badly. 4) If you discuss sex with B, a) make sure either both of you are present, or fully disclose anything that was said after the fact, and b) don't let her get the wrong idea about your discussions. She needs to understand that, while you both are definitely intrigued by the possibilities, you have made a firm decision and that she needs to respect that.
Hope this helps.