Yes, with Everyone it doesn’t matter, but for Staff then it will check the current user is in the staff group (which, unless you impersonate an admin or moderator, they won’t be).
Yes there is a hidden experimental_impersonation_time_limit_minutes setting set to 15 minutes. I’m not super familiar with this system, but definitely would be a nicer experience if we auto-refreshed/updated the UI once you pass this limit, or at least indicate it somewhere.