We'll see how this plays out, but as an attorney my opinion is that A&M has a much stronger case than you currently understand. To add to my previous point, it is important to note that according to the Seahawks, it is the fans and the media and the not organization itself that are off and running with the "12th man" concept.
If A&M wants to sue the fans for trademark infringement (which is certainly not beyond them in their current state) then they can have it. Suing the organization (if the Seahwawks are being honest) seems to be pointless.
and A&M said they have no plans to sue the Seahawks. I would imagine that they are allowing it to ride until after the Super Bowl at which point the Seahawks front office will probably remove their organization from its use voluntarily. I would doubt A&M would sue the fans, they are concerned about the marketing aspect such as the flag on the new stadium and any merchandise.
Further more what is different for A&M from UT suing the City of Fort Worth for using a logo similar to the burnt orange longhorn logo or Paul McCartney suing major restaurant chains from their wait staff singing Happy Birthday(which caused other organizations to stop as well). I would imagine that the A&M wouldn't even bother if it wasn't for some Alumni protesting. But it a trademark and should be defended to some level.