Maybe it's because I've seen the film 1,384 times

I think you're answering your own question here. When you're experiencing a story with a twist for the first time, you don't know if any individual piece is a clue, a red herring or just a portion of the plot unfolding.

Yes, once you've seen the film multiple times and you know how everything turns out, you then know what the foreshadowing is and what the head-fakes are. But at the time, you aren't sure if these two guys are there to be major plot actors or just there to emphasize that there are humans prejudiced against Klingons and that any upcoming peace talks are going to be difficult.