You're using a symbol fill, right? You have to edit the actual symbol used for the fill. Open up Symbol Manager, and check the "Show Fill style symbols" option. Now you should be able to find the symbol in the list, and you can edit it from there.

If you don't find the correct line style, it is easy enough to make your own. Just click the line style indicator, and make a new style. For a dotted style, you'll want very short line segments, and a distance between them that is still quite short, but at least twice as long as the line segments. And make sure you don't use paper scale.

Yea, as I mentioned above, you cannot edit a multipoly properly, which is why this should be the last thing you do.

To edit the multipoly, you need to temporarily disable it.

1. Hide your walls to avoid accidentally messing with them.
2. Use :CC2EXPLODE: on the floor entity. You'll notice that your "hole" disappears, but that piece of floor we used to make the hole in the first place should still be there, although it may be difficult to see.
3. Show the walls again.
4. Do your edits. (Note that we did move your walls to other layers/sheets, so make sure the oultine entity of your drawing tool is updated with the same changes, or it might not work properly)
5. Hide the walls
6. Use :CC2MPOLY2: again to combine the floor with the hole.
7. Show the walls.

That desert? edge screams for a switch to smooth, and a little bit of edge fade to make the transistion less harsh.

As you yourself conclude, the main issue is that many of these things are defined as percentage of view width, while the actual preview window is a fixed size, so this can happen with extremely large maps like this.

So in short, nothing is broken, just a view issue.

You can actually change the size of the preview windows, if you hit that advanced button in the drawing tool window, you'll have access to the sample width setting, which is the size of the preview, just bump this up a few notches. Do note that this is a per-tool setting.

At this stage, the bitmaps may look more pixelated than desired too, so you may wish to use :CC2RESMODE: to force a higher resolution for the display (Note that this can affect performance)