Very good breakdown of the situation. But cold comfort for anyone not in the USA. Because we know once Apple ships this it will be expanded to other countries with similar CSAM laws … many of which do not have robust Fourth Amendment type rights enshrined in their legal systems.

This is true, but there aren't many countries which Apple would even consider risking its global reputation in order to retain that market. US, China, Europe, maybe a couple of others.

To begin with, there's no prospect of this being done in secret. So we're talking about Apple complying to demands being made in public.

In the case of the EU, it would have to be passed into law in a way that makes equal demands upon all of Apple's competitors. And in that case, everyone must comply with the law irrespective of whether any existing CSAM detection apparatus was in place. (Thus it's a sheer cliff, not a slippery slope.)

If Saudi Arabia or Sudan tried to turn the screws, the business case for Apple is absolutely clear-cut: they would leave. It wouldn't even be up for debate. There would be far too much at risk globally than they'd ever hope to gain/retain in that domestic market from compliance. Not only would they never risk serious damage to their global reputation (something they'll be extremely sensitive to, as the last two weeks have taught them) it would represent a massive opportunity for Apple to earn weeks of free media coverage that aligns with their security narrative.

National Security Letters or their foreign equivalents routinely come with gag orders.

That’s a lot of assumptions of good faith within publicly known constraints