Wow, this is a tough situation overall.
I think that first thing that really stands out to me in your post is that you didn't state why you went snooping in the first place. What triggered that?
It sounds like there are already underlying trust issues on your part, which means there are likely similar issues on his side. (which is probably why he didn't tell you about the video or admit to having sent it in the first place).
May I ask how your relationship was doing prior to starting a quad?
In no way do i mean to sound judgmental, but I do know that many people turn to poly relationships when their own mono relationship is falling apart or seriously damaged in some area.
Poly is not a bandaid. If anything, it is a higher stress environment. So if your relationship with your hubby wasn't already strong and stable when you entered in to the quad, then what's happening is likely just a reemerging of pre-existing issues.
Even if that isn't the case however, I think it might be time for you and your husband to take a "just us" month or two and make sure you have worked out every aspect of your own relationship. The danger in not doing that is that if this situation isn't delt with in honest, open, heartfelt communication, then i may get worse. And if it does that, it will almost certainly drag the other half of your quad in to the tangle and that is not fair to them, nor healthy for the whole relationship.
Best of luck *hugs*
With all my heart I will love and not fail,
With all my soul I will fly and not fall.