[amazingman]

>what are the odds that Apple would accept a solution that was encrypted on-device?

We probably agree that the odds hover just above 0%

>would Apple still block the interoperation with their network, and do we agree on whether they'd be wrong to?

We could agree in principle that they would be wrong to, _if_ there were a clear path to continuously verifying that a third party client is behaving above board. Unfortunately that's just not the case. AFAIUI, this is still an intractable issue with encrypted communications. Is it an impossible problem? Probably not, but the amount of sustained effort this would require from Apple (and the 3rd parties) seems unworkable. So given that reality (I think), I don't think Apple are wrong to disallow third party clients for their E2E encrypted service.

>The way Apple's messaging has addressed it still leaves a bad taste in my mouth, because they do not make clear that what you point out is the issue

Yeah, I don't disagree here. I can only say that this is par for the course when it comes to Apple's PR. They would basically never explicitly state that their E2E encrypted service is at risk of a MiTM attack due to third party clients. Instead we will get very generic language and be left to fill in the blanks ourselves.