"If you could bypass the OS's password because the OS is broke, all those accounts behind the password manager would become accessible."
No, because the passwords are encrypted by the password manager.
"I've never forgotten my OS's password, but people must forget them, right? I very much doubt that totally bricks a device, which suggests there's a way in for technical types to reset the password when the forgetful take their devices in to be put right again."
With centrally controlled systems, yes. Because they had admin rights to the authentication servers. For your home PC? You're into a reinstall situation.
Edit: And in any case, getting past the OS wouldn't give you access to the passwords, as the Passwords Manager won't let you in!
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No, because the passwords are encrypted by the password manager.
"I've never forgotten my OS's password, but people must forget them, right? I very much doubt that totally bricks a device, which suggests there's a way in for technical types to reset the password when the forgetful take their devices in to be put right again."
With centrally controlled systems, yes. Because they had admin rights to the authentication servers. For your home PC? You're into a reinstall situation.
Edit: And in any case, getting past the OS wouldn't give you access to the passwords, as the Passwords Manager won't let you in!