I like your theme here very much. It would give you lots of interesting room for development. As for the voice software, I do think it can really help people understand how a piece will sound all put together. Even among professionals working in the arts, not everyone is good at visualizing a finished product, so anything that pushes a demo towards accurate representation of the performed work can be helpful for people checking out your work. How much work and expense is it to put this extra layer into the recording? For something like this, where you have orchestra plus soloists and choir, it may be more worth it than for smaller works, because if you didn't, it might be hard to choose an instrument to represent the voices that would stand out in the mix, particularly for someone who was trying to read across the full orchestral score and pay attention to everything at once. Looking forward to seeing the development of this project!