Will you have checked bags? That always complicates standby.

If you do, claim the bag in customs and DO NOT drop it off at the connections desk in T5 w/o seeing about being confirmed on the 4:50p flight. Sometimes agents can confirm you right away and retag your bag.

Don't get so worked up over it. Ideally you take the 4:50p flight, and if there are seats, you will probably get on. Keep in mind flights have been VERY full these days.