The Color Key effect can work with any color you want. Magenta (Color #6 in the CC3+ palette) is used by default since very few maps would contain this color naturally, but it can be changed to another color if you need to.

Nice map, but the background repetition steals a bit from it. I would have tried either changing the scale of the background fill, or replace it with something that isn't so noisy, or maybe fade it out a bit (can be done by placing a polygon on top with one of the solid xx fills)

Mercator is honestly one of my favorites. Of course, it doesn't go with Schley at all, but since you are thinking different map levels here and not mixing them in the same map, I don't think that matters. I imagine my different maps and map levels being made by different cartographers, and don't have an issue at all with them being different styles.

But there are so many pretty styles out there, and most of them works fine as world maps, it is usually more about limiting detail level when you map rather then a specific style needed for world maps.

These packs can be used in community atlas maps. All of the downloads presented in that thread will be set up so that they will be easy to get a hold of and install (correctly) so everyone have the same setup.

Basically, this means that CC3+ cannot find the image files used in the catalogs. The catalogs contains references to image files on disk, and a typical problem when you move computers is that you didn't put these image files in the same location on the new computer as the old one.

If you made the symbol catalogs using relative paths, it should be enough that the location inside the CC3+ data directory is the same as on the old computer, even if the directory itself is in a different location, but if you used absolute paths in the catalog, the path to the images must be exactly the same as before.

I wrote a blog article about it here. I talk about maps there, but the exact same applies to symbol catalogs (there's no technical difference between symbol catalogs and maps)

Let us know if this helps you figure it out, or you need more help.