I really think that to a large degree, this conversation is lacking communication.
Dinged-what I hear you saying is that you are emotionally spent, trying to hold everything together to deal with the kids who are emotionally traumatized (at least one of them), minimize damages for them and struggling to find the energy to deal with the job of helping your wife-who created this disaster.
I think that's perfectly reasonable and love/marriage or not, it's not your job to prioritize her over the kids-imho. The kids are your first responsibility and you've taken that on admirably.
BDSM activities can be healthy.
Abuse can look a LOT like BDSM activities and unfortunately only the "players" know for sure.
As you've said several times, you really have no idea. Maybe she was a willing participant, maybe she got herself into something she didn't know was going to fuck her up. I don't know either.
BUT-I think you are doing the right thing by venting your emotions somewhere the kids aren't and by prioritizing their care.
As for dealing with the kids-you didn't ask me but I have 2 cents based on experience.
I have a 19 yo girl, her father and I split when she was a baby.
15yo stepson, his mom is a crack addict and alcoholic (in recovery at the moment, April 9th will be 1 year).
11 yo son with Maca-he's a dream, parents... poly and into some BDSM.
3yo daughter w/Maca-bio child of GG, parents poly and into BDSM & GG is my slave.
I don't keep photos of anything questionable (including "vanilla" sex). Maca and GG do. They have their own laptops that they keep that shit on, it's not in "internetville".
BUT-if my kids encountered anything at all in photo, word or action (they have walked in on me during sex with one or the other guy)... I would deal honestly and up front with them, explain what was going on to the degree that they inquired and reassure them that it in no way impacts THEM or THEIR safety.
YOU can't do that-cause you haven't a clue what the hell was going on-if I were stuck explaining to my kids something I didn't know-I'd be stuck much as you are with "I love you, I don't know, I'm sorry you're hurting." Because, that would be the honest answer.
As for keeping secrets from my kids. I don't do it. I was upfront with my daughter that the reason she couldn't see her dad except supervised with her grandparents or me, was because he was irresponsible, drinking and partying and it was dangerous for her. That i loved her, I knew she loved him adn that was ok, but it was not ok for me to be neglectful by allowing her to be in situations I knew were dangerous-even with someone she loved.
I was upfront with my stepson the same way, your mom is on crack (skipped the whore part, he figured that out on his own) and she's a drunk. She drives while she's drunk and high, that's dangerous and it's not reasonable to allow you to be in that environment.
WHEN she went to rehab-I was the ONLY ONE who stood by her and supported her. She made a choice to change and I was all for it-for his sake. But I never glossed over the b.s. before that. Including being up front with him that she tried to kidnap my daughter twice, that she was filing bogus paperwork accusing me of b.s. I wasn't doing, that she was exposing him to people who were abusive etc.
My younger son-knows all of the above about his siblings other parents. He also knows their other parents.
All of the kids have been told honestly and openly everything about our situation, dynamic etc, except the details of our sex lives-which they have no interest in. They ARE aware that I am sexually involved with both men, but details-that's just gross to them. Which is FINE by me.
But-if one of them asked me "do you do xyz" I'd answer honestly.
I don't believe that keeping the truth from a child is a good idea, especially when that truth is that their other parent is NOT OK. It's possible to be honest about that and still be compassionate and supportive of them loving that parent. We all fail to meet perfection, chidlren do NOT need to be allowed to believe that their parents are perfect. They can learn that it's ok to love their parent AND that their parent REALLY FUCKED UP.
If it were me, I certainly wouldn't badmouth your wife, but I wouldn't undermine your daughter's self-esteem by telling her it was all ok either. REGARDLESS of whether or not the BDSM was consensual-it was NOT ok that she let it get to the point where her child had access to that shit.
I have participated in activities that were painful (not like you describe and I do NOT do humiliation) but tehre is NO WAY I would do that or record it in a fashion that there was a RISK of my kids accessing it. Not done when they are here, NOT RECORDED in writing, picture or video etc. Not acceptable.
When you have kids-you lose privileges, it's part of the responsbiility of being a parent.
Ok-that was a rant from hell. Sorry D. I'm still thinking of you!
"Love As Thou Wilt"