I believe they have two alternates -- one "referee" who can work any position except umpire, and one umpire. They may have changed this OR I may be delusional and they've NEVER done this.

In regard to Seeman, I THINK you're thinking of the AFC Championship Game (1987?) Denver-Cleveland. Tunney was the Ref, and I think back judge Dick Dolack pulled a hamstring and Seeman was the field judge for the rest of the game. He called Ernest Byner's fumble.

Everyone knows...
 

Everyone already knows...

Bernie worked this past weekend and therefore is all better to ref.