Okay. So I went back to the basics, and redrew everything from the mountains up. Here's the new (and in my eyes improved) mountain distribution. I think I found a fun way of visualizing the effect of the terrain on the coastline. Try imagining a world full of water. the bottom is made of a (permeable) canvas cloth, which is pushed above water by mountains. The trick - I think - is to draw the mountains in such a way that the canvas would indeed rise up from the waters and create the coastlines as they are on the map. I believe this attempt better succeeds at this than the previous one.
Still fidgeting with the rivers, so that will be for later. What do you guys think?
Coming up next: more mountains (southern continent), and rivers and shading!