You would probably just need to say that the SkookumScript plugin is a dependency. As I mentioned above, the marketplace team might be worried about needing other plugins though it should technically be possible.
You can store Sk scripts in any folder: the engine, plugins, a project or anywhere. You just need to ensure that the project .ini file has overlays (libraries) that point to the correct folder location.
That should be fine.
All of this will probably have some gotchas, though it should be possible.
You can, though you would have to add the bindings to the UE4 editor C++ calls. We actually plan to do this at some point in the future so that you can use SkookumScript to automate the UE4 editor. We just haven't had the chance yet since we've been focused on game runtime functionality. (The UE4 team at Epic would really like SkookumScript UE4 editor scripting too.)