I would be very surprised if Iberia let anybody without a flexible, cash ticket fly on an earlier flight on a standby basis. At the airport the ticket desk won't be able to change an award ticket either. Ground staff at that time of night will be wanting to get home, not do anything helpful.

If you're flying with Avios, I would just obsessively check availability every day and see if you can grab the seats ahead of time.

The Iberia lounge at Terminal 4S (most, if not all intercontinental flights leave from here) is open 24 hours, so you should be fine with spending the night there, provided you have a boarding pass. There are lots of decent hotels near the airport for less than 50 euros, so I personally would just book a hotel and consider yourself extremely lucky if you don't use it...

Note that I'm pretty sure you won't be able to go back landside once you are in T4S.