Ah good points, I wasn't thinking quite completely about some of the ramifications of direction of the subduction.

Thinking more about Silvertear... the whole lake seems to be surrounded by small mountains, so even if there is no outlet or inlet, could it be filled just as a giant rain or groundwater reservoir? It also seems that the situation in 1.x was that the Falls had pretty much dried up. They were labeled off on the west side of the map but no (surface) waterway connected them to the Lake. Compare this to the intro movie showing their state before the Battle of Silvertear Skies where it was practically Victoria Falls in scale.

And it might be possible the central island where the Keeper of the Lake is is a direct consequence of the Keeper' formation. I watched through the intro movie again and it's hard to tell whether the Agrius is touching down on land or if its pure water, but its conceivable the crash itself and resulting aetherial explosion uplifted some of the surrounding land.