> The real challenge is generating a real image that could be mistaken for CSAM at low res + is actually benign (or else just send CSAM directly) + matches the hash of real CSAM.

Why do you have an idea that image have to be benign? Almost everyone watch porn and it's will be so much easier to find collisions by manipulating actual porn images which are not CSAM.

Also this way you'll more likely to trigged false-positive from Apple staff since they aren't suppose to see how actual CSAM looks like.