First, no one will bat an eye on a one way ticket. Those of us that are experienced Southwest flyers do it all of the time. Just do not do it at the last minute and pay cash.
Buy an Anytime fare, fully refundable. Once inside the secure area, call Southwest and cancel and be sure to request the refund. You can even buy the ticket, at 24 hours before the flight print your boarding pass, then immediately cancel the flight. Use the BP to get through security. OK, this is a bit sleazy, I would keep the flight open until I am inside the secure area.
If HA will give you a gate pass, take it.
As for the TSA, they will generally take anything that is same day and not past. The standby excuse is OK, but just buy a ticket for a flight that leaves close to the arrival time.