I have a concern and a suggestion for sing-along assignments.
Have there been any issues assigning non-existent vocal animation parts to band members, assuming a song has less than four parts? Will they instead use the same animation as lead vocals?
Instead of giving the user an option to assign vocal parts to band members, how about giving them an option to assign band members to vocal parts (including part1); do it backwards. Considering your concept, doing it this way makes it easier for the user on which band members sing along with lead vocals, and if more than one band member shares the same non-primary vocal animation. Plus, it makes for a good space-saver than having duplicate animations via Magma.