S.B.
I just went to my 18 yr old daughter's college orientation with my ex-husband (my husband from my 2nd divorce - we have 1 daughter together) - we stayed in separate rooms, but still ate together and hung out together. We even snuck off campus and had a few drinks together. We have vacationed together in the past, we have eaten at each other's homes, he has come to family functions for all 6 of my kids like birthdays, graduations, dance recitals, etc. My 2nd husband is a great guy. I like him as a person. My daughter know that her dad and I are friends - she also knows that we are not meant to be married to each other. We have been doing this for YEARS - from the time she was a little girl. I don't think she ever said that she wished we would get back together. Why would she? Things are so much happier than when we were married and she always felt like she had the best possible family situation.
As long as everybody is up front and honest about what is going on, I think you should go. I definitely would NOT bring up the fact that you are only doing this because it was already booked. I would play up the idea that even though you aren't "together," you like and respect each other and look forward to still doing family things together.
Good luck!