I haven't read this whole thread, but I've read all your posts, TiF.
You've gotten some good advice, especially IMO from M1965.
I'm writing to point out that you're not even 2 months out.
My guess is that you would not necessarily make a decision that affects several decades of life and that is not urgent in that short a time. If it takes a couple of more months to make your decision, it will extend the pain of not knowing where you'll go, but more time will likely get you a better decision for you.
Given the probable impact of your MIL's suicide on your W, given that you don't necessarily want to D, given that some of your W's attitudes support R, given that you have your head on straight about boundaries ... I think you will benefit from not pulling the plug yet.
You are not showing weakness, IMO. Rather, you're maximizing the likelihood of getting what you want.
I'm not a big poly fan. I think human beings do better detecting lies than a machine does. Your W committed to scheduling a test, though, and it's important that she meet commitments, but I think her new job commitment is much more important.
Honesty is required (and I mean that literally) for R & M, IMO. If a poly becomes essential to you, then it's essential. But I have no doubt that you can become better than any polygraph in differentiating honesty from dishonesty.
And even that may not be essential to R, given that some Rs have worked with professional contact only.
Bottom line, given that you're less than 2 months from d-day, you're in much better shape than most BSes.
*****
She says our values don't line up about a lot of things (e.g. politics, some ethics, etc.).
You're 2 different people. You won't agree on everything.
BUT ... hiding her sister's infidelity hits very, very close to home, and you know about it. Neither of you can hide from her sister's A.
In your case, I think MC is entirely appropriate, if you use the sessions to help you resolve the issue of revealing the A and your other differences in general. MC can be a very useful venue for deciding how to go forward.
To R, you'll both have to change some. Your SIL's A is a great test case to see if you both can do the necessary work.
Personally, I see a much greater likelihood of successful R if one or both of you cause the A to be revealed or reveal it yourself.
I can understand your sister not wanting to out the A, because she was told probably in confidence. You were not told in confidence. One option is for your W to tell her sister that you overheard her and that if she doesn't confess within 10-14 days, you will out the A.
Immediate confession is better, but - again - this isn't urgent, IMO, though it certainly is important.