DG,
First step, while opening the laptop or phone make sure it is not connected to any internet connection so it will not sync with the latest cloud and you can loose a lot of info. From the laptop, you will retrieve more likely naked photos of her/him. I don't know if she is using an outlook or anything like that, than you might have the whole history of her emails. If not, you need to check the history of her browser, you can get the dates of all the emails sent or received with no content as probably if her email was a web based everything would be deleted by now.
On the other hand, the phone will be the killer gun. You can retrieve all her messages even if they were deleted. There will be voice messages and maybe short MMS as well. Whatever, she was trying to hide, you can come close enough to know the dates, their schedules, when they were meeting if there was any other motels scheduled or any parking lot sessions from the messages.
One more question I have, have you got your land line and her phone bill details. Can you do that. It will be another helpful item to verify.
You need to get all these prepared before the polygraph. I know at this moment you only want the truth and you can not move an inch without it.
Whatever information gathered after she gave her passwords, take your time, prepare all the information, than you had retrieved and confront her in front of all the family that are backing her now and let her explains everything in front all of them especially your daughters.
The polygraph should be after that. As she will be lying in front of them as I figure. Once your break her walls and see her transparently, it is up to you to decide if you can allow her to R or D. It will be your decision not your daughters, not the family, it will be yours.
As I have seen now, she has dragged you into her plan. Well it is an opportunity that you can take it to your advantage, but not the way you are acting. You have to draw the boundaries and time line. She and all the remaining of the family have to follow it. Just be calm, decisive, and let everyone knows that you are in control (don't lose that). Prepare the topics for the MC, should be about her infidelity and not your marriage. Preferable, if you can chose the MC and gave him brief alone before you attend together. NOT her rules....YOURS. She can manipulate both of you. Obviously, she has her story ready by now and she is convincing everybody around. (could be true) She is well prepared, you need to keep it in your mind and act accordingly.
She has not helped you at all till now, she was manipulative. You need to break her walls by the truth of her reality it is than when you have the chance to R in the future if this what you decide to go with. Don't forget if she had cared for you a little bit, she would have come clean by now, and not drag you with her bullshits and make you
suffer more and more without knowing the truth.
I know by heart that nothing can be hidden for ever, as I have seen on multiple threads that secrets were revealed after 30 - 40 years. However, is your life worthy to live the lie for the rest of it.
Good Luck, and God be with you.