I'd turn down dinner as well. If it were my STBXW offering I might say yes because there is no OM in the picture.
In your case, it's just him trying to string you along.
You apparently have a good relationship with the in-laws. It's just a MIL left for me and she's OK, but I wouldn't do anything with her or any of STBXW's family.
To me, if STBXW is divorcing me, they all are divorcing me.
Me: 47, Ds 17-13, D final 6-11 http://tinyurl.com/yk4e2tz http://tiny.cc/thread2 http://tinyurl.com/ydtphqu http://tinyurl.com/thread4 http://tinyurl.com/3sm78k6 http://tinyurl.com/thread6