Good that there’s some human supervision. But, I know I have more than 30 photos of my dog. Also don’t like the idea of false positives auto-sharing some of my camera roll.
It's only if you back it up to iCloud, the signatures of the CP used as references are rotated, and they're also not public. The chances of you randomly triggering the system is effectively 0 unless you're uploading CP to your iCloud.
No. They calculate a hash on the device, but they only do it as part of the iCloud upload. So whether the hashing happens on the device or on the server, the same images get hashed either way.
Photos of your dog are not going to trigger it. Someone would need to engineer the 30 photos of your dog tweaked to hash to a particular value, and then convince you to save them to your device and then upload to iCloud. And then some portion/abstraction of the dog photo would need to convince a reviewer they were looking at CSAM.
The more likely path to trouble is legal NSFW material that's been engineered.