It sounds like you dont need to be too concerned about the OM much as he is a spineless worm that can be controlled by your STBXW. It sucks that your D13 despises him. That in itself does not sound like a good way to start a trip.
If your W is not telling you about OM going on the trip, then that is just another thing she is being sneaky about. Let her continue to be sneaky as it is not worth pointing out to her. I think as long as you can be friendly with your STBXW then it will allow for the two of you to move on through the divorce with minimal pain.
Yeah, She's been with him almost a year and the kids are used to putting up with him. D13 is the one who has to deal with him the most and she just ignores him when he's around. D18 just doesn't go to her moms place very often, and when she does OM isn't there.
I don't think she is being 'sneaky'. For some reason she won't tell me directly she is doing things with him. Yet she told me months ago I needed to 'get used to her having a boyfriend'. Is that weird or what? She hasn't 'introduced' him to any family members. Her mom will be the first, and is the only one who supports her decisions to leave me because she did a similar thing to her dad 20 years ago.
And of course, she supports her daughter who has suffered so much being with me because of my depression and drinking issues. It's funny how everyone forgets the good parts. Oh well.