Checking ExpertFlyer, it appears the cheapest West Coast FI flights book into W while the East Coast flights book into S. W is not listed on the AS Partner charts for Icelandair while S is listed at 50%
If flying from the West Cost consider AS to YVR to connect to FI. The cheapest fare also books into S (50%) and the base fare is lower than SEA/LAX.
ie November 7th, one-way base fare, SEA-KEF, is USD $237.00 fare class (W)ODLT no AS credit. YVR-KEF USD $183.00 fare class (S)ODLT 50% credit to AS.
Fees & Taxes will be higher out of YVR. If you connect with AS to YVR you will earn the minimum 500 EQM.
All in, SEA-KEF $335.40 Earns Zero MP miles
All in, YVR-KEF $380.91 Earns 1772 MP Miles
Add SEA-YVR for $94.83 Earns 500 EQM.
James