Grimrock

Shipwreck Beach has those awesome semi-open caves (picture in spoiler) and I'd like to do something similar, but can't find any assets in the editor resembling those overhead rock shelves. Am I missing them, or are they in the asset pack, perhaps?

As for the asset pack, how do you actually use it? I've searched Google and the forums and there doesn't seem to be anything on this. The tutorial on custom assets seems like it might apply, but it's not clear whether you can copy-paste assets/scripts from the asset pack into a project or if you have to manually edit Lua files to include the assets.

The asset pack is just the standard assets that are already in grimrock2.dat. There is no point in adding asset pack files to your mod because they already exist in the /assets/ directory. The asset pack is provided so that you can look at & edit the definitions, models, textures, animations, etc. to learn how they work or to help produce custom assets.

The cave is just a combination of several standard objects. Just look at Isle of Nex in the editor to find out how it was made. Here is a dungeon editor file that will load Isle of Nex.

The caves are made with the "beach_rock_arch", the "beach_rock_arch_small" and some "beach_rock_walls" beneath them.

Your secound question is unclear for me.
The assets from the main-campaign (and some more) are all included and can be used with the editor. If you have questions about whats in the asset pack it is a good idea to download it and have a look at it. AFAIK nowhere the assets from the pack are listed. If you want to know how things in the main-campaign have been scripted you can grab it, too and have a look at it. AFAIK it is no problem to use code from the main-campaign.

That answers my questions. Thanks guys!