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"Is there a way to tell from the Alaska Air website if it is the lowest tier?"

Only if you know how many miles AS requires for its lowest-tier awards on a particular route.

The easiest way to check is to look on aa.com. Any AS award flight you find on aa.com is, by definition, a lowest-tier award flight, and should be bookable with Avios.

Note that AS flies SEA-EWR nonstop; AA flies SEA-JFK nonstop. You can redeem Avios for one way on AA and one way on AS. But you would have to find lowest-tier ("SAAver") award seats for those flights on aa.com.

If necessary, you might also look at redeeming Avios on CX's late night YVR-JFK or JFK-YVR nonstops. But there will be some carrier surcharges when redeeming Avios on CX, plus the cost of getting to or from YVR.