Do not plan another activity around the time of the reunion and definitely do not go to any part of the reunion or meet up with the relatives when they are in town for the reunion. It will end up backfiring on you in some way.
You and dh have made a choice, to set a boundary with mil. In this case it means not attending the reunion due to her not respecting your wishes, even when she said she would. You have made a great gain and do not want to lose it. If you go to any part of the reunion, mil will not see that as you doing what you want, but as you really didn't mean what you said and you will have the same type of situation happen again in the future.
Any time you make a choice there are consequences. The good consequence in this case is that mil, and by extension fil, will know you are serious and will follow through with your boundaries. (Thus you cannot show up for any part of it, meet up later, plan your own activity.)
The bad consequence is that LordL will not get to see his relatives at this year's reunion. It sucks and it doesn't seem fair, but that is part of the decision that you are making. If you want to make the point with mil, you have to also accept this consequence and not go, see them any other time or plan an event. It will be tough, but you will get through it and the benefits in the long run will be sooooo worth it.
You will see a lot of them at your wedding. maybe you can also make plans to visit them separately at a different time than the reunion if time, money and distance permits.