Bumping this thread as it's the most relevant to my question. I couldn't find a definitive answer. Some articles I read said an Iceland Air stopover in KEF requires a round-trip ticket. Specifically, Mikhala Stulzman in this article says it will not work with a one-way. I have done some experimenting with fares and I see no reason why a one-way stopover doesn't work. The outbound one-way is slightly more expensive than it would be when paired with a return flight, but it seems as though FI allows one-way flights. Anyone have thoughts on or experience with this matter?