My suggestion is to go there alone with new clothes, nice facial appearance, and small dab of cologne.
You can acknowledge W in a friendly manner. As for MIL, you an say "hi, it's good to see you. I hope FIL is feeling a bit better and please give him my regards." Then go about your business. Don't offer to bartend alongside her.
The one question we never, never ask our WAS is 'how are you?' because they feel that you're intruding into their private space and their wall will go right up. Then you'll lose your chance to showcase the fabulous Nit. You can say "hi" to W and comment tHat you hope the fundraiser is a success because those people will need everyone's support.
Now, I want to be gentle here....you need to mentally prepare yourself for the possibility that W may bring OM to this event. In her mind, W most probably feels she's "single" which means she is 'free' to bring him around. Not saying that it'll happen, if ever.
The goal is to mingle and be your charming self while attending to the friends you're supporting....which is the purpose of the fundraiser.
