As for another sitch where telling people outside of the immediate loop works or is in the process of working...
My H told his parents and some guy friends of his. He told me that he "recommended" that i talk about this with some of my girlfriends before they found out "some other way" because that would just make me look worse. I talked to them about it. While they all were pretty upset with me that i'd do something like that, they all were very supportive of us trying to work our M out. He did proceed to talk to them (my friends) to see what they knew. I was honest, so he didn't have to tell them anything, but he would have. I was upset at first, when he first told everyone. I was thinking this was just our business, but I know why he did it. It's not his fault i chose to have the EA. He shouldn't have had to hide my dirty little secrets...
b50 - IMO, in regards to MC and dinner w/ dentist and anything else that comes up. I recommend being very honest and straight forward with her. Tell her that you would like to try MC once she is done having (whatever sort of) R with OM. Tell her that you are not okay with an open M. That is not an option for you.
If you let her walk all over you, she will continue to do so. Tell her what you are willing to put up with and make very clear what you will not put up with. Stand up for yourself and set your boundries!
If i can't fall in love... I'd like to fall in chocolate! ~ Author Unknown