It isn't normal for all iPhone 5's to do it. That was confirmed to me by Apple themselves. The rep told me not all iPhones are affected.
That rep was probably telling you that in that hopes that your replacement phone would be accepted through placebo.
My thoughts? I think to some degree they all do it. And by "some degree" I mean of greatly varying degrees. I showed my photos to a coworker who has the 5 and she told me she hasn't experienced the issue at all.
A lot of it has to do with usage and how/where the user is taking photos. Some iPhone 5 owners are taking indoor shots, or framing away from the sun like most photographers should, and will likely not experience the issue nearly as much. Others, like yourself, will be actively trying to take pictures under conditions intended to aggravate the issue, and of course you're going to get
exactly what you're trying to see.
There are probably some minor variances from one sensor to the next, but not to the wide extent you claim.
Or maybe she just hasn't noticed it yet? Either way, I don't think it should be happening.
Purple fringing is well-known, long-established artifact of digital photography on a lots of digital cameras, not just the iPhone 5. It's not a desirable thing, but it is a limitation of the available technology. That's all there is to it.