4th - Advice on how to get this to run on Linux?
I have limited knowledge as to how StepMania works under-the-hood, but as long as you don't do anything hard-coded (in the executables and library files), your project should just be drag-and-droppable onto a Linux version of SM4.
I haven't looked at your project yet, but it seems you're only working with a theme and noteskin (basically, visual stuff)? If so, I don't see that being too much of an issue dragging right onto Linux.
The main question is how well SM4 runs (if at all) on Linux... Also, any particular reason why you aren't using SM5?