An image being erroneously flagged will not have any effect. Until the threshold is reached, Apple is not able to know if or how many images have matched.

And even then, the voucher doesn’t include the key to decrypt the photo itself.

Apple has documentation that explains all of this.

I doubt it, since those photos will likely be close to porn, ie. of naked people and Apple doesn't want people to know that the naked pictures they took of their 22 year old spouse is now floating around some random office in Bangladesh.