I don't think the Dragonslayer is actually Ornstein, he uses some of the moves but clearly not all of them and he doesn't have quite the same affinity for lightning and fast movement. He is definitely old, judging by how sluggish he reacts compared to the Ornstein of olde. I reckon it's more a honorary title than anything else, since there are no dragons anymore, the last of the dragon slayers died or disappeared long time ago.
Something else I wonder... who it she lost sinner? And why is she fighting in a style which is resembling moves from Orstein? In Dark Souls 1 Sif fights in a similar manner as Artorias does, so does that mean that the lost sinner has a connection with the now long dead dragonslayer?
@Geography: I don't think the sea was rising to get Anor Londo under water, rather than Anor Londo sank into the ocean like Atlantis. Having a look at the map
, the Area Heide Towers stick out of the sea looks almost like someone just took a bite from the coastline
But now I don't think Heide is Anor Londo itself, since the geography doesn't make sense that way. The mountains separating AL and Sen's Fortress/Undead Burg are missing/wrong if HT is AL. Hence it's more likely that Heide was built by those who abandoned Anor Londo when the fire started to fade and before what happenend in Dark Souls 1.
I remember there was a reference in the first game that a lot of people have left, abandoned the gods. Heide is German for heathen, which I take as statement from FromSoft and would explain their lack of gods in the tradition of Anor Londo, why people forgot the names of the gods and why the faces have been replaced with those of animals representing ideals rather than showing statues of actual people.
Heide Towers is the city that those who abandoned the gods of Anor Londo built far away from home, centuries ago. A city which flourished and was home to machinery today's Knights of Blue can't understand anymore, and which sank into the sea long time ago.