That valve did open the opposite way it should and allowed fumes to enter the living space?? Didn't you also say earlier there were two slots in addition to that valve?
I may have to revisit what mine looks like. I also had the same thing happen which is why I took mine off in the first place last summer.
On another note regarding the use of an exhaust fan, if no window or other vent is open I could easily see a backdraft coming up when the toilet is flushed...
