Even as a loyal OW customer, I opted for Icelandair in Saga class when I went to KEF. It's not worth the extra time and cost to connect in mainland Europe and backtrack to KEF, and the Icelandair onboard product is good. Plus, the Icelandair lounge in KEF is nice, and, depending on the airport, they have partnerships with other carriers for lounges in the US.
FWIW, I wish Icelandair would become a Oneworld Connect partner.