Hi. I am glad it works!
So to your suggestions:
I thought about using roman numerals but decided against it as I want the program to be usable by people not too familiar with music theory and notation. Also, If I went from arabic to roman numerals they would change when you switched from major to minor for example. (IV would be iv). Also, if I went that route, I should really add notation for dimished as well, and it would quickly become confusing.
Your other suggestion about being able to use your own progressions are actually exactly what I am working on now for the next release. I am also working on giving the user more direct influence on the different parts of the music, for example, if you find a drum pattern or bass line you like, you should be able to keep that and randomize the rest.