As to the passage of time, I recall that the A occurred over a period of around 3 months, plus or minus, when your son was starting first grade, fall of 2016. This means it must have begun some time around August/September, 2016, and you discovered it and confronted her some time around November/December, 2016. Like around Thanksgiving/Christmas.
I hadn't thought about it that way before. The holiday season must be Hell for you. We've discussed previously the concept of "what Thumos do you want to see in the mirror in 10 years?" Narrowing that concept, this will be your holiday companion, for life. What version of Christmas Eve do you want to live for the next 10 years?
This also means that the time they had sex in your home (the one you know about -- more on this below) must have been around Halloween? Kids at a Halloween party? Parents snick off for a little extracurricular?
When was the period at which he made her feel like no man has ever made her feel before? The thing that you now can't understand because your "immature" views about sex prevent you from accepting that it's okay for her to feel these feelings about a man who is not her husband?
As to the anxiety she appears to be experience about the poly, common wisdom here suggests it's because there are fundamental truths she hasn't told you that will need to either come out, or be reflected as a lie. It strikes me as entirely possible that she is planning to continue to lie to you -- for example, about the number of times they had sex -- fail the poly, and chalk it up to her chronic anxiety.
As to the MIL blaming you for the "bad marriage", I'm reminded how she had re-written the history of your marriage leading up to the A. It seems that your WW and her mother have a way of reinforcing one another on this to construct a shared trope that you are the problem in the marriage, and then your WW uses her expert gaslighting skills to cudgel you with that theme. Even now, post-A, where she is staging some anxiety theater to set you up to disbelieve a "false" on the poly question of "did you have sex with him more than once?"
You might remind your WW and your MIL that, when it comes to anxiety, you live in a hellscape life where you are forced to personally encounter the AP regularly, often several times per week, and each time you see him you are reminded that your wife harbors a secret cocoon of intimacy with him: kissing, touching, sucking, fucking, pillow talk, even in your own marital bed. A cocoon that she has steadfastly refused to open, that she protects against the marriage, which leaves you gasping with anxiety each and every time you see him, wondering what piece of your WW he still owns.
By the way, if the timeline includes specific dates of texts or emails, and quotes, you ought to ask how she could know that. Either she has a cache of this material, or she has asked the AP for info, because there isn't any way a person could recall detail like that from fall of 2016.
[This message edited by Butforthegrace at 6:09 AM, November 6th (Wednesday)]