If you can forgive him and not get an immediate OMG he's sick and I can never live with him kind of feeling, then why wouldn't you talk to him about it? Question: Do you trust that he wasn't having sex with her the whole time he was vacationing with her?
Obviously you're not going to leave this guy so why be afraid to talk to him about something that important? What are YOU afraid of is the question you have to ask yourself. Saying you don't want to open it up after such a long period of time doesn't really clear up what you are afraid of. If you were'nt afraid of the answers I think it would have been a normal human response to be concerned and to ask whats the deal?. iMO and for your own peace of mind, it's important to know his thought process for such a thing happening but also because he's kept such a video for all this time. (that to me is a real concern because it indicates he's been viewing it and reliving it.) When exactly did he think he was deleting it?