This is really cool. I like all the mountains on the Targonian Plate - it gives some rationale for why the middle part, south of the Vestigial Plate, would be a giant desert.
A recommendation: drop the Vestigial Plate. Rather, have a mid-ocean ridge run down the middle of that sea. That ridge would then be the dividing line between the two plates. Long ago, the Targonian and Valoran plates would have been connected, but they split apart, forming that ocean. For comparison, see the Red Sea or Atlantic Ocean on Earth. Now, I imagine the Rune Wars or some other magic-related catastrophe might have accelerated that process, which would have been really traumatic for the people living on that side of Runeterra.
Have you looked at wind patterns or ocean currents yet?