2 - DO arrive within the time window and plan times around other plans.
The return window on the FastPass selection is one full hour. You can arrive anytime within that hour and ride the attraction. You definitely want to make sure that you arrive on time within that hour. Disney used to allow a few minutes buffer on either side of the time window, but usually Cast Members will not allow you to access the attraction if you are early or late. You want to get there on time and not bank on a cast member letting you in after you missed your window. Sometimes they can’t do this due to the popularity of some of the Disney World attractions, and you definitely won’t want to miss out. Each attraction has a special FastPass enterance. A clock is always part of the enterance sign so you know exactly when you can enter. If your attraction closes during your return window either due to inclement weather or technical problems, you will be able to return at a later time to ride the attraction. The new return window information will be available in your My Disney Experience App.