Hi left. I haven't actually had an SO in rehab, however from reading many posts here at SR, no, your story is certainly not unheard of. Rehab romances, people getting attached to someone in their group. These people are looking to fill a void, whatever route that might take depends on the person I guess.
I had delusions of her sobering up and us working through this
I don't know if this is really a delusion, but of course hope. As it is right now, I would squelch that, she is non-communitive and really just using you and your Daughters to support her life choices. She is with this other man. If she was honest and told you where she is at - that's one thing - that also hasn't happened.
Yes, recovery is a selfish process just by its very nature, however that doesn't mean a person needs to be rude and uncaring all the time, which, from what you have said, she is. How long has she been in rehab?
Not looking forward to the holiday season
When was the last time that you did and did she participate? It's only October! I understand this is stress and grief talking, but perhaps one day at a time is a better approach right now?