What about the fact that A&M didn't get trademark rights until 1990?
This is a very technical distinction, but an important one. A&M got rights in the use of the mark as soon as it started using, not when it registered the mark. This is what a lot of people don't get. The only thing registration does is provide constuctive notice to other potential users--i.e., Seattle is deemed to have first knowledge of A&M's use of the mark no later than the time of registration. You can only acquire rights in a mark if you do not have [actual or constructive] knowledge of another's use of the mark (assuming the marks are in fact confusingly similar).
Further, the fact that several other NFL teams were served CAD (cease and desist) letters strengthens A&M's claims and weakens any Seattle may have.