As far as I know, all airlines with change fees are somewhat sneaky with re-booking you. Iberia is not unusual in that regard. Changing a flight incurs the change fee (usually $275 in the US) plus whatever increase in fares has occurred since the time you bought your ticket. That usually means that you pay a LOT for rebooking. I have in fact known people who just buy another round trip ticket and use half of it to go home. The cost of two RT tickets is frequently much lower than changing your reservation on your original ticket.
I think the hands down cheapest thing is to pick your return date and buy a roundtrip ticket, leaving in as many wiggle room days as you can. Then if you have a lot of days left over at the end, you can either walk to Muxia and/or Finisterre, go visit some other place nearby (lots of great spots to visit in Galicia, Asturias, Portugal, etc.), or walk a short camino such as the Inglés from La Coruña, the Portugués from Tui, or the Vdlp from Ourense. Those of us in the US are at a real disadvantage when it comes to flexibility, unlike the Europeans who can just reserve a trip home a few days out of Santiago. I do this every year and always build in what I think are four extra days, so I can then walk to Muxia-Finisterre if I'm right or just slow down a bit if I'm wrong. Buen camino, Laurie