> Yes, but the key is stored in your iCloud backup if you use it. As soon as you disable iCloud backups it will roll the key for iMessage and they will be effectively E2E encrypted.
Assuming this is true, you still don't know what people on the other end will do, meaning it is never actually E2E encrypted.
E2E usually means from endpoint device 1 (my iPhone) to endpoint device 2 (my friend’s iPhone). What the other person will do with it doesn’t factor into the conventional definition of E2E.
No, conventional definition is actually both: from endpoint device 1 to endpoint device 2 and from endpoint device 2 to endpoint device 1. If device 2 has backups in question enabled, there is no E2E anymore.
Assuming this is true, you still don't know what people on the other end will do, meaning it is never actually E2E encrypted.