Last's years annual may be of interest as a starting point. It contained two issues dedicated to graveyards and crypts.

This is a dungeon style though, but if you are going to a zoom level where you can actually see such details, you kind of end up there. For larger scales, city, and especially overland, such details tend to disappear.

As far as I know, there aren't any necropolis styles for overland or city, but perhaps some of the darker styles may be appropriate to provide the right mood, like Darklands and Darklands City.

The exact issue here is basically one of calculation when things get small. At the scale those maps above are exported, there isn't enough pixels in your map to make those "cover walls" in your map a full pixel wide. Unfortunately, pixels cannot be partial things, they either contain one color or another, they cannot be split, as they are the smallest unit in an image. So, when that happens, the rasterization process needs to decide, that line is less than a pixel, should I round it up to 1 or down to 0? One might thing rounding it up to 1 is the good thing since you do want them visible, but for every time you do that, you steal pixels from something else in the map. So when something is less than a pixel, it will sometimes be 1 pixel, sometimes 0. So in short, some of those cover walls just disappears because there isn't room to show them.

But where really makes your problem here is that outline of the shaded area of yours is outlined in a 0-width line. Those lines are special in that they are always drawn as thin as possible, but they are always visible. So they are always one pixel no matter the zoom level. So when that wall hiding line disappears because it is too small... Well, that outline below it won't, because it is always that small.

So, how to deal with this?

One way is to simply export the image much larger and then reduce it afterwards. As long as the covering lines had enough space on the export to appear all of them, they won't magically disappear when you resize the image afterwards, it is just a case of making sure they are given room during rendering. This can be done by exporting a higher version and resizing in an image editor, or you should be able to do the same by setting the expected size in CC3+ and use a really high anti-aliasing value (probably need 100% if you really want your results as small as in this post) as this will export a much larger image and then scale it down to compute AA.

You can also try covering them up with other 0-width lines instead of wider lines. I am fairly certain that would work fine. As long as things are placed using snaps, you can easily place another 0-width line exactly on top of it. And that zero-width line should have the same behaviour as the line below and allways show, and allways be exactly large enough.

The error stems from that map not having a background color.

This might immediately sound a bit weird, but in CC3+, a bacground color is associated with an actual entity on the BACKGROUND layer, while the color you see in the map isn't the background color, but the view window color. One way you can see a difference between the two is that a background color is only behind your map, while if there is a different view-window color, you'll see it behind the symbols in the symbol catalog window as well.

In short, you don't want to use the Change Background color on that map. You'll want to use change properties to change the color of the coastline, and View -> View-Window Color to change that dark "background" color.

The structures aren't defined as Hex symbols, and I believe that is intentional. Hex symbols in CC3+ has some special properties, one being that they replace each other so there will only be one in any given cell. This enables quick mapping and easy changing of features without having do delete stuff first to avoid ending up with a stack of symbols. The mapping guide for the style also mention that they are not true hex symbols.

Unfortunately, that also means they won't scale automatically, so you need to manually set the correct scale before using them (although it should normally remember the scale if you just placed a hex, but if it doesn't set it manually). But once you have set the scale correctly, the snap grid for the map should allow you to place them correctly aligned on the hexes.

I did notice your snap grid was wrong, and after checking it seems like this is a problem that occurs because you set the grid from the new map wizard. Unfortunately, it can't set the snap grid from there for some reason (notice it is greyed out), so life is easier if you make the map without a grid, and then just add it immediately after creating the map from the wizard, as this allow the correct snap grid to be created. Not sure why it is like that, but at least adding the grid afterward is an easy workaround. (Or, you can just edit the snap grid manually afterwards, that works too)

Not sure why you can't change the colors here. I went through the instructions and it seemed to work as described for me.

Which color is it you have problem changing? The color of the costlines, or the background color?

I don't know about any videos or additional instructions, unfortunately, but the data is quite detailed, so you should be able to cut out the pieces you need and not use the entire world after importing it.