Sure, but if someone in the middle wants to store, then they don’t have anything unencrypted to store. Ultimately it all depends what one defines the “ends” as. In general for backup, both ends are the user. For chat, the sender and recipient. Apple is not an end, but a provider in the middle.
User B screenshots all of those messages and uploads them unencrypted to a random FTP server.
Now replace the random FTP server with iCloud. Or Google Drive. Does that change the nature of the messaging between User A and User B? I would say it doesn't. In fact, WhatsApp does basically the same.
If you think the word "blame" is too morally hot, then we can always go with causal attribution.
b) this is just false; whether the data is at rest or not has nothing to do with the definition of the term. And it is misleading in any case.
b) The etymology of the term has been blurred by WhatsApp referring to it as both transport and encryption at rest. But historically it has not included the latter part.
No, it's not because it is another instance. Someone can have one thing right and another thing wrong. And whatever the true meaning of "E2EE", its use in this case is misleading at best, so I'm pretty sure that the profession does not approve of it.
The user is being used as an excuse: "we compromise your privacy because that user over there wants it". This is not blame in the obvious sense of "it's that user's fault that they lost their data", but it is blame: "the responsibility for this thing isn't ours, the people who wrote the code, but yours."
b) End to end encryption has always meant transport encryption not encryption at rest.