This is not entirely clear: when an event makes the affected car move back, is the safety zone established based on the position of the affected car before it moves back?
(That's what we assumed.)
You just need to follow logical principles. Resolve the event card first and then institute the Safety Zone. So if a car is moved back on an event, it is resolved. Then a Safety Zone is placed. Therefore it would be placed where the car is after the conclusion of the Event, not before.