I mean, don't get me wrong. I have a bottle of champagne in the refrigerator, and if Thumos comes here Monday night and says she passed the poly, I will pop the cork and drink the whole damn thing and celebrate into the wee hours (put this on the Google calendar, TimeSpiral).
But it's my New Year's champagne, and I don't think I'm in any danger of having to replace it. Why did Neanderthal's wife agree to a poly she knew she would not pass? Because she thought the truth would kill the M, and she hoped against hope that she could find some way to avoid admitting it. That he would decide to believe her. A liar trying to avoid a poly by agreeing to take it is nothing new under the sun.
I just don't want you to have any false hope here, Thumos. And I don't want you to fall for her backup plan, which could be to go ahead and fail it and then either (A) blame it on anxiety or (B) tell you one more piece of TT and say, "That's all now, that's everything." Then you'd have to schedule another poly to confirm that's really it, which she is pretty sure you won't do, and in any case, she still has "they don't always work" as an ace in the hole.
I want to mention one more thing, since I'm being such a Debbie Downer. This is your life and your marriage. It is not anyone else's. No matter what you say you're going to do if she fails it, you don't actually KNOW what you're going to do if she fails it, and you won't know until Monday night, and possibly long after that. And that is your right.
My concern is just to be sure that you make your calculations with a clear head and not a pipe full of hopium. If she fails the poly, it will be because she is continuing to lie, and about serious things, not little details. When that happens, you may find that despite everything you believed about a "package of non-negotiables," you don't want to break up your marriage. You may make the call she's banking on, that your family life matters more to you than the truth does.
I think that would be a mistake, because the uncertainty has tortured you for three years. My BH had to know everything, and now he does, and a lot of it was devastating, and it brought him as low as a person can go. But he will tell you that the worst things he learned are things he can heal from, and the unknown was not. This is after 30 years, not 3. For some BS, it just does not go away.
But that's me projecting. Your marriage is yours. Your choices are yours. The power is yours. You can do anything you want after the poly, whether she passes it or fails it. Everyone here, including me, will be chock full of opinions, but you will make the call. And it does not have to align with anything that you told a bunch of strangers on the internet.
So hang in there, Thumos. One way or another, you're about to learn some very useful information. What you do with it is still up to you.