I'd like to pick at that "haze/turning into a zombie" connection in the light of your slow turner theory. What if the haze is the thing causing people to turn, but the speed at which it works depends on tour exposure level? If the sole global source of the haze is in
Ground Zero, and if it's been venting at low levels for an extended period, say a week or so before the day of the Outbreak, then tens of thousands of people will have been exposed at low levels and gone about their business. Inglewood is not far from LAX and two major freeways. Exposed people would undoubtedly move on to the rest of the US, Hawaii, Japan, the Philippines, everywhere that was effected by the initial outbreak.
On the day of the outbreak the volume of the haze goes up to a critical level, causing people to turn rapidly. All that's missing is some signal mechanism to activate the infected.
The slow turner at
the Colony would remain an outlier, since this "Trojan Horse" tactic depends on the infected transforming rapidly. I'd guess he's someone with a high level of natural immunity to the infection.
One big downside to my idea is that it makes Victor, Saul and Pegs ticking time bombs, since they've been exposed to the haze.