This is a very interesting thread, now in my mind there were three incidents that could have been "turning points" (probably a lot more, but these 3 are the ones that stand out for me).
1. What if at the "Trooping the Colour" in 1981, it wasnt blanks, and The Queen was in fact killed?
Would being made King just about a month before the wedding have given him the ability to marry Camilla instead? Probably not with the Queen Mother & Phillip still alive. Would being King have made the marriage to Diana easier or would it have propelled events must faster with the extra strain of reign?
2. What if Charles died in a similar "blanks fired" event in 1988?
We would probably have never known much of the problems in the marriage, there wouldnt have likely been a morton book, or panorama, or dimbleby book. But at the same time, I think Diana wouldnt have felt the need to strike out and find a role as much, so things like the landmines campaign wouldnt have happened, shed have had her hands full raising the boys, and a lot of the problems she had with Charles would have been gone, so she probably would have settled into a widower role, albeit still a famous one.
3. The current one in the thread.
That one would have seen Diana continuing her new role making a massive positive impact on the world for her humanitarian work. Her personal life would be less certain, perhaps time and wisdom may have aided her there, but I fear it would have been more of the same. She likely would have channeled her energy into the boys.