Impossible. Because iPhones randomly force you to enter your iCloud password, often at the worst possible time. Such a user would have already been prevented from signing in to their own iMessage on a random Tuesday afternoon.
One of the most frustrating issues with Apple's UX.
It depends. It sounds like you're all in on the ecosystem. I only have the apple account because I have an iphone. I didn't get one of those double check reauthentications since many years ago, up until last month when I got apple tv. I know because I had to reset my password and it was older than my use of a password manager.
I've never experienced that. The only time I've entered my password was when I set up the iPhone and when the phone got into a weird state after a software update.