I will be on an American flight bc there were no United flights out of Fairbanks. Can I use my club pass while in Seattle even though on American? This will be in June 2019.

I'm a United Club Member and you need to have a booking on a United flight, or at the very least a Star Alliance member airline (American is not). They don't check your membership, they check your boarding pass - the membership is already in the system with your booking. But, since you don't have a United booking - nothing is in the system.

I would think if you're a United Club member, you can use any club regardless of airline you're currently flying.