As described so far, the system is on device or out to Apple's private computing cloud. If the system thinks it can use OpenAI then it will ask. It will ask _each time_. So far Apple has not said that there will be a complete opt-out but I expect there will be just as there is for Siri.
"Out to Apple's private computing cloud" is exfiltrating data off the device. It very much should not be lumped into the same category as "on device", and should in fact be placed in close to the same bucket as "sent to OpenAI". I don't know the details of how it works, but from your description it sounds like you expect sending your data to Apple would not be opt-in?
Homomorphic encryption is not practical. If you're sending data to a server for processing, that server must be able to see your data. If someone tells you otherwise, they are lying.
No they are not.
As described so far, the system is on device or out to Apple's private computing cloud. If the system thinks it can use OpenAI then it will ask. It will ask _each time_. So far Apple has not said that there will be a complete opt-out but I expect there will be just as there is for Siri.