The most important questions at the outset, IMO, are those posed to the MC: what is your experience with infidelity? What is your approach to guiding a couple post-infidelity? Etc.
In terms of what to ask your WS--well, I'm a firm believer that any or all questions you have deserve a full, transparent response.
I would want, therefore, to be sure that I did not choose an MC whose approach to recovery from infidelity erected any barriers to this.
Unfortunately, there are rugsweepers among MCs, just as there are among the rest of us. My questions would be geared toward identifying these and weeding them out.