I think you need to use the time until your W's return very wisely. If it does not endanger your job, take the next days off to get stuff sorted.
You must use the worst-case-approach because you owe your children safety. Meaning: assume for the moment that everything you saw AND suspect is true; that there is no other "good" explanation for it and that most likely this is just the tip of the iceberg.
Than ACT accordingly. If, against all olds, things turn out differently, you have not lost anything.
You do not have to understand your wife right now. Your shocked brain is not eben capable of that, so don't invest energy in comprehending. This can come later. My guess is your wife suffered severe sexual abuse by men very early in life and a part of her brain equates that mistreatment with love. A lesbian relationship gave her the space for a new, unthreatening beginning, but as she never dealt with her trauma, her inner child resurfaced, craving the sick affirmation it was conditioned into longing for.
This is all extremely sad, no doubt. BUT IS NOT, UNDER ANY CIRCUMSTANCES, YOUR PROJECT OR RESPONSIBILITY. You are indebted to your children, then to yourself and only then to another human being, eben if your heart tells you otherwise. Also, she can only heal herself. None can do that for her.
Now is NOT the time for fixing your W or your marriage.
Now is only time for damage control.
First, find someone you can trust, both with your kids and your evidence. Your kids CANNOT be there when you confront, whenever that is.
Also find someone who can be with you, when you confront, as witness and in case it gets violent.
Important: This first confrontation is not between equal partners who try to truthfully communicate. THIS WILL NOT HAPPEN. Your wife is about to lose her safe space, that place, where she could uphold the better version of her self. In the first run, she will do ANYTHING to keep this space; then, once she realises its gone, she will be full of hatred and rage towards herself, which will then turn unto you. Finally she will most CERTAINLY try to emotionally blackmail you with your children.
Give up the hope of a good and honest conversation NOW. It may happen one day, but it will not happen now.
Therefore, confrontation should only take place, once you have done the following:
1.See an attorney. ASAP. Ask beforehand, if the initial consultation is free (it should be). You can (after the first interview, not before) then offer to pay in installments.
2. Find someone who you can confide in, who can take your kids and the evidence and where you could all stay for a few days. SHARE THE EVIDENCE WITH THEM, even if it is hard.
3.If this is not possible, find an orgsanisation that could help for free, for example one specialising in domestic abuse. They will help even if things where not physically violent (yet). DON'T BE ASHAMED. There is no time for shame now.
4.Collect all evidence, links, videos etc. Make screenshots. This is not complicated, google it. At the very least, simply make fotos of all videos, conversations etc. You need to have all that before she can delete her accounts!!! Best would be, to find someone tech savvy. Make a photo of where the phone was hidden. If you cannot get the phone to run again, take it and hide it outside the house. Do not be manipulated in giving your W the evidence.
5.Open your own bank account NOW and have your next paycheck go there.
From what you write you have to step up your financial literacy in the long run.
6.Find yourself a therapist if you can afford it. ASAP.
You are a beautiful person. Someone so caring and loving that even your broken wife could feel safe with you. You deserve happyness. Don't forget that!