I cant recall what the window is for the cancelation before it becomes more of a hastel...I think you can do it online about an hr before the flight/

If you are unsure of making the first leg have you booked an alternate flight on southwest? They dont do standby stuff

If you are not sure if you can make the 5pm flight but you know you can do the 8pm flight you book these all as one way tickets. Then once you know which one you can take you cancel the other one. and it goies as credit for a yr from the purchase date.