First, how about some proper spacing so your wall of text is more digestible. But concerning your issue, the world isn't one world. Solaire tells us in our first encounter with him that our different worlds phase in and out, and last only a short while. It would make sense, then, that a phantom would phase out of your world and return back to his proper world upon defeat.
As for there being more than one chosen undead, well, I like to think that eventually your soul begins to fade, and a new chosen undead must one day usurp you. In this unending cycle, you even replace yourself in NG+. Is the Gwyn we face even the same Gwyn of the old legends? Of course this is all speculation, but in my head it makes sense.
And are they really immortal? When you die you lose a bit of your humanity and your souls. Eventually wouldn't this drive everyone to becoming hollow and thus mortal?
Second, Manus lures you in to Oolacile by taking Dusk as bait, otherwise why didn't he just kill her? Why would he be interested in her at all?
He wants your soul because it's so powerful. The arena where you fight him is very much an arena keeping the abyss at bay. Notice there are no solid walls, more like a magical barrier. We can assume he doesn't want you lost in the abyss.
Third, why can't we assume you're badass enough to kill a dragon? You are the chosen undead after all.