1. - You are invited so your wife can tell you there, with a third party present, that she is leaving.
2. - You are invited to work on things. She will present her issues with you, don't get defensive, listen and validate.
Either way it doesn't change your goals, boundaries or values. Let the MC know what you think about your marriage, the behavior going on that is bad and that you are still committed on your side.
M22,H45,W45 S21/18D12 Retain faith that you will prevail in the end, regardless of the difficulties and at the same time confront the most brutal facts of your current reality, whatever they might be.