Stu, I would make a list, I know that it sounds cheesey, but make a list of OK topics, so that in an awkward silence, you dont slip and start talking about off limits topics. Discuss anything BUT what you told her you wouldnt discuss. No relationship talk. Keep things positive. If she tries to bring something up, either politely change the subject or politely leave the conversation.
Look and smell great, and dont be too hard on yourself. This isnt THE conversation that is going to save your M, but it is a good chance for you to show her your new positive mental attitude, and that you are serious about your boundary (telling her that you wont talk about divorce or separation).
If you dont think that you can maintain your PMA or keep from discussing D or separation, then you maybe shouldnt go...
I guess I gave the wrong finger to the wrong man...