On the topic of Ancient soul fragments and reactions to the starshower, I definitely don't think that everyone has a fragment of an Ancient's soul inside of them, for the simple reason that there's simply too many people. Remember that you have to include all the beastmen races along with all the human races, and there's suddenly an awful lot of Ancient souls to account for. Add to this, the Ancient population was DECIMATED prior to the Sundering - there just weren't all that many Ancients left to have souls to split.
But, assuming for the sake of argument, that everyone DOES have an Ancient soul fragment within them - what happens when one more child is born? Is that child then born soulless, because all the Ancient souls are already in use?
No, I think most likely there's only a very limited number of Ancient soul fragments around, and the only difference with them is that when their bearer dies, the soul doesn't dissolve into the Lifestream as regular souls do, but instead floats around and is eventually reborn into a new host. It is likely these hosts who are the individuals capable of expressing the Echo.
(Note that I might be wrong on this, and the writers simply haven't done the math. There was a discussion a while back on the topic of whether the population of Sunseeker Miqo'te is even sustainable with only twenty-six tribes. They could well be an endangered species!)
As for why non-Echo individuals might still be moved by the Starshower, the tragedy of the Ancients may well be marked indelibly into the Lifestream itself, so ANY lifeform born from it might feel a twinge upon witnessing the Starshower.