Did she ask or indicate? They are two totally different methods for invitation.
It appears that the truth is you two have been separated for a substantial time and have both moved back into the dating world. With the recent struggles with our daughter, you have had to re-team completely on the co-parenting front. I have gone through what you described (the therapy part) and though the objective is your daughters welfare, it was also an unpressured bonding experience. There was no thoughts of relationship, etc only the common TEAM WORK associated with a common problem. So then what is a marriage or a relationship...basically a team of two people to accomplish a common goal.
I would take her up on the offer. At minimum you get to spend an extra holiday with your daughter. I agree with most of what is said above, but at some point you have to decide that all interactions don't necessarily have to be controlled by DB etiquette of darkness and LTR.
