What you present as evidence for your claim is much more likely a result of one of many other factors. Every FPS excluding Halo has center targeting that I have seen, yet many of those FPS's are not nearly as popular as Halo is.

As far as CE being the most popular Halo game, that is debatable, but I don't have numbers to support anything so I won't continue on that route.

The popularity of most game series wanes after a large number of releases, and also considering that a fairly large amount of the fan base couldn't handle the more noticable changes in Reach's multiplayer, I think there is a much much more plausible explaination for this point.

As for the last one, I don't know how to respond to it, considering just how vague it is. It seems far more likely that people would respond negatively to a much more noticeable visual stimuli, like bright and or strobing lights, as well as brighter, and luminescent colors(all of which have been known to cause epileptic seizures in some), rather than the simple position of a small circle on the screen.

It is far more reasonable to assume that other factors caused these things that you consider the reticle position to be a factor in.

As far as the H3 flood infection is concerned, it is possible to prevent the reanimation of the flood that you kill with a head shot, by disposing of infection forms, the two situations are not the same.