I think saying you will not do so and so because of OM could be read as holding on, waiting until OM moves out so I can take his place. A Plan B. Just saying NO thanks without any resons I think is better. Leave out the "because" add endum... NO reasons. Let her guess why and if she asks just say you dont feel like it or similar. Don't let her see there is an emotional reason behind.
As the rest of the guys are saying, do not ask questions.
Listen to your S, he is telling it to you straight.
If she calls you and you feel the urge to pick up, do so. Then say can I call you back and dont. If she calls again say sorry you completely forgot.
I would also stay away from family gatherings. If the bridge between you and them is your W, if she fired you, that bridge is burnt. They are no longer your IL's. Now is not the time to think about them, let her deal with the vacant chair. You have a vacant heart to deal with.
