D - My first thought was no. Do not go. She wants to be single, but has no difficulty playing the part of happy family when she doesn't want to stand out in her chosen role. That is so unfair, it makes me irritated on your behalf.
On the other hand...the kids. Dad being around for Halloween would be huge for them! So I would say go. Do not let her act like you're a couple, though. Be friendly, but keep some distance. Focus on your boys and your friends and you should be okay.
I would talk to her about the birthday so you get some idea what she's planning. Your boys are so young, I hate to think of them having two separate parties. That would just drive home the situation for them on what should be a fun occasion. Find out her thoughts before we start working on that.
