Arguably. I don't see it as such though. I see it as: if my phone would be fine in a deposition then I don't have to worry about work spying on their device. But if I have to worry about what work might find out on a device then I'd be a lot more worried about what might show up in a deposition.
What's said in private is supposed to stay private. It helps to demonstrate that it has stayed private if it can't be compelled especially from unrelated circumstances. Whether "private" means discussions with your family, your friends, your coworker, your boss, your commander, your patient, your client...
...it's very important to keep work separate.
They will, and you won't. (I've been there.)
No, they'll want to see the communication. Which the company will be compelled to provide. The device isn't relevant. (In my, admittedly limited, experience, this is how these processes actually work)
No, this isn't correct. If opposing counsel has evidence[0] that the personal devices have relevant communications not already provided, they can file for a subpoena to have you turn over personal electronic devices. Your counsel can file a counter-motion, and you'll probably have a hearing.
No where will there be a warrant, unless you're in criminal proceedings.
Like I said already: as far as I know, litigation holds and discovery are fairly common, and what you're describing is not. To clarify, this is all assuming you use a work profile and a personal profile.
[0]: I don't necessarily mean evidence in the legal standard, but they need some basis for this. "A feeling" isn't enough.
Wait until your personal phone, with its work profile, comes into scope of a work-related deposition.
You'll find just how paper thin that argument is.