You've worked hard on it, and it's a pretty good start :)

Ralf did a Live Mapping session on DD3 lighting, which you can actually use in city or overland maps, though its not so common. This example is dungeon, but it should be fairly easy to adapt. Shout if you have any difficulties.

https://www.youtube.com/watch?v=TQKEswmn10A

That doesn't sound right.

Have you run Tools->Actions->Burn into surface?

It's a while since I wrote ODW, but I think this is necessary before you can do rivers well.

It may be that you have too much fractalisation going on, since those kinds of artefacts tend to happen when there are nodes too close together.

If you have thousands of nodes when you use Info List on the entity (just to make sure I'm right about that), try using the keyboard command SIMPLIFY to reduce that node count. Even if you use quite a small factor the command will at least remove the nodes that are causing the artefacts.

If SIMPLIFY is the answer, delete the wall or the floor that you didn't simplify, and use change like drawing tool to replace the missing part identical to the simplified shape.

To make something pop is to make it stand out, or up in the case of walls. Some people lengthen and deepen the shadows to make things look taller, though this can be a bit misleading because it makes the whole map look as if the building has no roof whatsoever. Some people play with the glows, especially a faint dark glow set to inner on the floors coming in from the edge, since it mimics the real world. Light never gets into the corners as intensely as it hits the floor in the middle of a room.