Switch-User function
What about a function "Switch to user" like used in VHCS2? Admins and Resellers have an icon "Switch to user" in the clientlist. By clicking this the interface is shown like the user would see it with one difference: A small line on top of the window with a text like "User 'admin' logged in as 'mustermann'" followed by a small log-off button to change back to the own user settings.