Check the FAQ; it answers the 'guarding' question and a lot more interesting tidbits.
Don killing the soldiers with guns: There is an entire prequel (14 Weeks Later?) that I'd like to see that shows the organization of these facilities, including the training of the troops as to what to expect and look out for. That might have answered your query: I don't know if many folks were taking cell phone videos back then in 2003/2007 that would have ended up on the nightly news or even in soldier training videos. Chaos.
The quarantine room: That's an interesting observation, but it was only Andy who escaped. I think when the authorities set up these things they figured adults would behave rationally. They blew it with Andy, here, as well as with Andy/Tammy when they skirted out of the zone to start the whole mess going. As Scarlet said when she saw the kids show up from the jet: Sheesh!
I don't reclal the scene of the soldier holding down his mic button.
Personally, I love the outbreak scene as it was exactly the kind of panicky mess that I wanted to see somewhere in 28 Days, but I guess they didn't have the budget for it - except maybe that choppy news footage at the very beginning, which may or not even be from the outbreak.

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