Use the trim tools, such as :CC2BREAK: to create breaks in lines. Combine it with the various modifiers, such as :CC2MODINT: to start/stop breaks exactly where the lines meet with the line from the other room.

You can then combine the line segments into one path using tools such as Combine Paths if you need to. (If you need to depends on how you are going to use them. Just having individual paths that just meet up in the exact same point is often enough.)

The reason you couldn't find them is because they don't have CA51 in the file name. If you look in Sue's screenshot above, you'll see that the 'Dunes CA51' fill references a bitmap called 'Dunes.png'

That's why you can find them using that dialog, but you need to do so from a map in the appropriate style. From there you can find their names, and then in the map you want them in, you can then easily add them once you have learned that filename.

Not much. Mainly a new short chapter describing the Symbols along, fill with symbols, and symbols in area.

Try a fill with a more visible texture. It gets a bit harder to find something nice due to the perspective nature of the map, but you could try with something like the Stone Lava Cooled bitmap from DD3.

Additionally, the pixelation of your current fill is really bad, causing it to appear more like a uniform mess. Try decreasing the scaling of the fill as well. That leads to more repetition when zoomed out of course, so you may need to consider working a bit with the surface, applying layers of different fills and blending them using effects to avoid the repetition.

The symbol behavior is down to you having lots of unrelated stuff on the HEX/SQUARE grid layer. Hex symbols are special symbols that automatically align with the hex grid, but all that other stuff is basically just confusing them.

As for the export, it looks for the lines on the MAP BORDER layer to see what area to export (If you have the restrict to map border option on), and if you check that layer, you'll see that the defining lines (green in your map) just covers the map area, and do not extend to include those informational areas.