This looks great! Always fun to see other worldbuilders who take their geology seriously. That story seems plausible, and we do have something similar with the Black Sea and the Caspian Sea. That southeastern archipelago you're working on looks great as well- I do love a good volcanic island chain.

On an unrelated note, you really must post a tutorial of your technique on the forum at some point- this is one of the only examples I've seen of a realistic-looking DEM on a large scale, and it's almost enough to convince me to figure out Wilbur at long last...