Straight up, OP; lots of misinformation in here. I can only offer an opinion on why this issue is so beleaguered with inaccuracies. When something "tastes good", we logically infer that it must be unhealthy for us. After all, we know that sweetness means sugar, and sugar is unhealthy in large quantities. Therefore, logically, things that are sweet are inherently unhealthy. This is why so many people think that diet soda is unhealthy, yet have no discernable reason for their belief. The most common response is, "Well, it's soda, of course it's unhealthy."
Fact is, Coke Zero is basically water. Diet soft drinks are all essentially water. If my memory serves, there is some concern that the amino acids present could damage the teeth over time if overexposed, by eating away at the enamel, but I do not believe that has been firmly established yet. A straw works well, or just tilt your head back and pour it in. Otherwise, there is no negative effects to drinking diet soda. Drink as much as you like; drink ten a day if you want. Not only is it perfectly fine, you're probably staying more well-hydrated than those people who are judging you and drinking 2-3 cups of water a day.
Just remember, there will always be a massive amount of misinformation surrounding this issue. You'll never get the peace of mind of a unanimous agreement, but you make take solace in the fact that the science is on your side.
Excelsior!