Leave them alone. You're doing a fine job of rationalizing why, but ultimately isn't your goal in doing it to effect your W's decisions? If you're honest with yourself aren't you hoping your MIL will sit your W down and tell her she's crazy? If the answer is yes then you're pursuing, albeit remotely. Instead of directly pursuing you're using them as your proxy.
I know it's hard. When my MIL stopped by on Halloween and gave me a huge hug I had to fight like crazy not to break down, not to cry, not to tell her everything that's going on. In my head I had this vision of her talking to my W and telling her how stupid this all is. But I stopped. I know that won't happen.
And if it's that you care what they think of you... why? Why do you care? I mean, seriously, what does it matter what they think of you... they're your in-laws and soon to be ex-inlaws. Talk about a group that doesn't really warrant much consideration. If you're going to insist on external validation then at least start with your own friends and family. The last place I would go would be my STBX in-laws and family.
If you truly are worried what they will think about you, then I would take that to an IC appointment for discussion. At this point they really shouldn't matter all that much.
Married 6 together 8 Me:38 W:31 second marriage for both SS12, SD10, S6 Bomb: 9/8/11 (day before our 5 yr ann) W moved out: 2/18/12 D final: 11/12/12 Share S 50/50. Spend as much time as I can with SS & SD