So if I understand you correctly, the effect you're after is a path snaking it's way upward on a mountainside, similar to what I did with Corgin.

In that map I relied heavily on the cliff texture from SS2, as I found it works really well. I also gradually darkened the path by using the Solid 10 Fill style, basically by drawing larger and larger polygons, so that the lower part basically has more layers, darkening it more. Since Solid 10 is almost transparent, the edges between the different amount of layers is practically invisible. I also drew in some polygons manually to serve as shadows from the cliffside.

If you are running a recent (less than 10 years old) 64-bit version of Windows, you don't need to do anything.

You can't set it at generation, but you can set them via the global operations afterwards. (Tools -> Global Set)

Yup, Gifts of the Old Ones.

All artwork in .png format can be used in CC3+.

Generally just put the .png files in a folder under the Symbols\User folder in your CC3+ data directory, and optionally import them into symbol catalogs.

Of course, CC3+ symbols are more than just the backing image, they have properties and configuration to make things easier, like houses that aligns to roads and doors that cut walls and symbols that automatically go to the right sheet. Artwork not made for CC3+ won't have all these properties unless you set everything up for it.