Setting admPanel.override.preview by default causes login and content errors
Setting admPanel.override.preview by default disables frontend logins and shows scheduled content incorrectly because the values for simulating a user and a time are forced as 0. We need admPanel.override.preview to ensure that hidden content elements are always rendered and can be controlled by the checkbox at the top.
Trying to work around this issue exposes how complex the relationship between timing and order of execution can be. In the end, the solution is to add an feeditadvanced-specific controller that extends t3lib_frontendedit. This controller can force the frontend into preview mode before any content rendering happens.