You don't have to tell her you talked to your MIL -- these would be conditions that you would reasonably have, as her husband, ANYWAY.
I'm suggesting to just use the info that you have from your MIL to know that your wife is warming toward you, and ask her for NC and full transparency.
If she doesn't want to do it, you'll at least have your answer.
You still sound absolutely TERRIFIED of losing her, or saying/doing ANYTHING that will "spook" her. She's going to be able to smell that from a mile away. You need to learn to lose the "desperate" vibe you're sending off. That's not a knock -- just an observation.
