Squiggle,

You are doing so well in Plan A! I know it is hard and I applaud you for your efforts.

Maybe I should just talk to them and tell them in general terms that we are having marriage problems, but that I still dearly love their daughter and in no way want a divorce -- that I want to fight for our marriage, but that my wife is not sure she is willing.

I don't see the point of this. You risk all the downside of full-blown exposure (W getting angry, telling you that the M problems are no one else's business, blah blah blah) without reaping any of the benefits that exposure is supposed to bring. The folks you expose to will most likely try not to meddle, and to keep out of it. They'll think you guys are arguing about what color of carpet to get, or whether or not you should get a dog. If you calmly and matter-of-factly tell them that she has moved out in order to have an affair with someone else, then those who care about her will become alarmed and perhaps speak to her about it. At the very least, they'll know and that will humiliate your W and take the bloom right off the A.

I absolutely see no reason for a half-hearted exposure.

JL mentioned something that is key:
ask the people you disclose this to for their help in saving your marriage, because that is what you are doing it for, not revenge.

When you expose, be factual and completely honest. Have the mindset that you love your WW and you want the assistance of others who also love her. She needs support and some guidance because she has lost her way. Don't go into it with the mindset of getting her back or making her hurt like you've been hurt.

Your underlying intent is key. You expose precisely because you love your WW and you want her back by your side.

turtle makes excellent points.

Here's another data point in the "never underestimate the deviousness of a WS" department:

Months before my d-day in June 2000, and not revealed to me until a couple years later, my WS was telling her family that she and I were having "problems" and that I was being "abusive." She was laying the groundwork for her planned exit. OM was doing the same thing on his side. So when d-day arrived and I sought assistance from the in-laws, I was an instant ogre. The table has already been set. They had been warned that I was going to make crazy accusations. According to plan, they did not (initially) believe my description of an affair and refused to defend me as a viable, reliable family constant in the aftermath of my son's passing - after they had witnessed up close our five year long battle with his cancer. Even after it was obvious that marrying the guy she "wasn't having an affair with" contradicted her denials, it was all acceptable because I was "abusive" and she didn't love me anymore.

Point: don't pussyfoot around with what you tell her family and the sooner the better.