Vantage Point (2008) - 3 questions

Question: When we're following Forest Whitaker he looks up at the "shooter building" right before the shot fires, and he keeps saying "No, you shouldn't be there." Does he know who it is? And who was it? From what I gathered there was no one in the building, and the gun was fired by remote.

Answer: He assumed by the movement of the curtains that there must be someone up there, which of course the Secret Service should have cleared to make sure there was no-one there.

Question: What was the significance of Rex's cameraman being in on the assassination plot?

Answer: It's not stated in the film, but if I had to make an educated guess I'd say his job is to make sure the news channel doesn't get any footage of any of the terrorist agents or the window the shots come from.

Question: In Thomas Barnes' perspective, near the end of it, he orders the studio guy to rewind the tape and then he says "stop", followed by "oh my god", but when the complete terrorist plan is shown, he instead stays staring at the monitor because he's watching Taylor. Is this a mistake, or are these two separate events? If it is the second one, what did he see in his perspective?

Answer: From what I remember, it's the former. When Barnes says "stop," he adds "rewind that." Once the tape is re-round, he watches it run through, which is then when he spots Taylor.