I'm not really experienced but I will let you know my opinion on this. Don't react to it, just tell her have a nice time and leave it at that. Getting upset over this in front of her will do nothing for you. You don't know the reasons for sure why she is going to this specific place so getting angry doesn't help you in any way. She could be trying to get at you, or she could just be a WAW doing things that don't make sense.
I understand your frustration. I've had a few similar experiences where W has done things without me. Things I wanted her to do but she never wanted to. Then some other things that we had planned to do together and never got around to doing(First tattoo). It made me sick to think she was leaving me behind, angry at the thought she might have done it out of spite, and just jealous in general.
Something else. "...I can possibly say something that would end our marriage for good.". Your looking at this wrong still. You don't say something that will end it for good because the marriage is dead. Your marriage has already ended. She ended it, not you. You need to accept this. Any future you have with her will be in a new marriage, and that may or may not happen.
Accept what is, let go of what was, and have faith in what will be
