Are you serious? Look at your OP, your site was yourself:
>Source: I am a former criminal defense lawyer.
Additionally, your reply is regarding CP - to my post which was about SR. In another post I replied to a question about CP, in which I stated CP is not strict liability (unlike SR), that possession is a much more complex analysis than people think, and even noted the "blind eye doctrine".
>However, it would not apply to the original situation--the witness does not intend to possess the video beyond the length of time necessary to get it to the police
Speaking of citations...
In 2007 I wrote a draft Romeo and Juliet Law for the Florida Legislature (did not get passed), but it was presented at various CLEs. It was tailored to protect minors from ending up on the Sexual Offender List for life (among other post conviction punishments of minors). To that end minors are charged and convicted in Florida regularly for receiving unsolicited mass texts messages of other naked students in their school and the charges generally start with the student who whistleblows and sometimes ends with the victim charged. The facts are similar to your Park hypo where the possession was unintentional and the material is generally immediately turned over to authorities.
You better believe anyone turning in any video that includes a minor having sex, no matter how brief or unintentional, will immediately be read their Miranda rights and if a prosecutor with ice in their veins wants to charge you, they have the discretion and they will happily present evidence of a video that includes a minor having sex and evidence it was in your possession.
CP is not uniquely excepted -- you must have the intent to possess the content that is CP for it to be a crime. If some other person acquired CP and left it on your computer (for example, in a real case but not one of mine, a grandmother's computer was filled with CP left by her nephew was not guilty of possession of CP), you are not guilty of possession of CP. I won a case in Cali on these very grounds.
BUT...what I think you are referring to is that you do not need to intend for the content to be CP. That is, if you make a video which is, unbeknownst to you CP, the intent to possess that video is sufficient. In that regard, it CP possession is similar to SR in that it is "strict liability" with regard to one of the elements. However, it would not apply to the original situation--the witness does not intend to possess the video beyond the length of time necessary to get it to the police...and indeed, destruction of the video could be deemed destruction of evidence!