Orthodoxy/praxy is a big deal when navigating these waters.

First and foremost, nothing's happened yet.

I'll quote from the hymn Do Nothing Till You Hear From [Them]

Someone told someone and someone told you But they wouldn't hurt you, not much Sine ev'ry one spreads the story With his own little personal touch

Do nothin' till you hear from me Pay no attention to what's said Why people tear the seam of anyone's dream is over my head

Overall they treat [wife] quite poorly and she knows this and it hurts her.

Sounds like you two have let some boundaries get weak. Gotta nip that in the bud quick. When they reach their greasy and inappropriate words across your boundaries, you tell them to knock that shit off. Every time.

Anyway, we're having our first kid in a couple weeks. She is worried that they will question everything she does as a parent that isn't quite orthodox enough for them.

This isn't exclusive to Mormonism. I think every grandparent in history has questioned whether they raised a child capable of bringing up a child. Sharpen your boundaries to razor precision and make the grandparents feel that it's dangerous to get too close to those boundaries.

Then of course there's the big one, what to do when they find out that I don't believe it anymore.

Do nothing till you hear from [them]. And set those boundaries.

[wife] worries because of the impending "I told you so..."

Your wife is still a believing Mormon; if they gave her this advice as pertains to you not being 'strong enough in the church' then wife has to grow up and take her lashings. She doesn't get it both ways. In other words, she has no other recourse as a believer than to feel that shame, and as a spouse than to feel that sting.

You owe it to your wife to tackle an issue each week until she understands your position. No laziness here. If she trusts your judgment in the very slightest, then she should be losing sleep at the possibility that Mormonism isn't what it claims to be.