As far as the "Frodo thinks he's dead" hypothesis, the important thing here is that Frodo is obviously saying "Gimli!" as soon as he sees the Dwarf (this can be clearly heard in the audio, too). This reaction would be expected to be somewhat different from the earlier ones, not only because Frodo isn't as close to Gimli as he is to Gandalf, Merry, and Pippin, but because Frodo assumes that the three people he's seen so far are dead. He'd have to momentarily process the thought that Gimli's also dead - not necessarily a surprise, but previously unknown. (As a person who's heard some early legends, he could also be thinking, "Ah-ha! Now I know what happens to Dwarves after they die!")
Pippin gradually loses out to the cropping here, but then he reappears. I've played around with the color balance and lighting in these caps - I'm not sure how successfully; a lot of effort seems to have gone into Frodo looking unwell in this scene. I can counteract some of the yellow lighting, but he also seems to be wearing make-up that makes him pale (except for the purposely dark areas around his eyes), which I can't do much about. I've posted what I've done, instead of redoing it, because the make-up itself is kind of interesting to see (most obvious around caps 12 and 13), and, besides, I'm too tired to redo everything!
18 (I want this tunic!)