You can’t cut or shorten video files with Cubase, but then you don’t really need to duplicate the video file each time, the one single file can be referenced by all projects just by dragging it into the project timeline each time.
As for audio, the same can be done too, you don’t need to copy it into the project directory you can leave it in a central place somewhere.
However, to actually shorten your audio I’d suggest you bounce it after you’ve shortened it in the project.
You can also create a kind of 2nd pool called a Library (in the file menu). I sometimes use this mechanism for files shared across a few projects (e.g. sound effects or specific samples in an album say). The library is exactly the same as the pool but it doesn’t have to reside in the project folder and it can be shared in more than one project. This means for you that if you create a library with the raw audio files and video file then in each project you can open the library and reference those files quickly.