I have the same issue here. I have an enterprise license, my staging is running on iis, I created a separated admin account for staging and staging is not logged in. And the anonymous authentication is enabled... I ran out the clue.
When I create my staging, I actually restored db from my local sitefinity db, I guess it will have the same sitefinity instance ID, Is this the same problem as you mentioned in your point 1? Otherwise, my staging is running on aws, could this be a potential problem, too?