The government need not care how Apple complies with the law, which could merely state that cloud storage providers are liable for illegal material stored there by their customers, regardless of the cloud provider's knowledge. This would be catastrophic to cloud storage in general, of course, but given that strict liability is a thing, I don't see how such a law could be ruled unconstitutional.

Trying to workaround the 4th like that might manage to make the law facially constitutional, but I'd be surprised if it made the searches conducted as a result of it valid.

By my understanding you have to avoid a "warrantless search by a government agent" to avoid violating the constitution. The "warrantless search" part is really beyond dispute, so it's the "government agent" part that is in question. In general "government agent" is a term of art that means "acting at the behest of the government", but I don't know exactly where the boundary lies. I'd be fairly surprised if any law that allowed for accidentally storing CSAM after a failed search, but didn't allow for accidentally storing CSAM content without a search, didn't make the party doing the search a government agent. If you make the former illegal, cloud storage at all (scanning or not) is an impossible business to be in.

You already have the FBI partnering with computer repair shops to do dragnet searches of customer's hard drives for CSAM when they bring their computers in for repairs.

I'd argue the Fifth Amendment should apply to mobile phone headsets, but law enforcement would pitch a fit to lose those.