(Not a Teacher)
While the terms make sense, I don't think I've heard them used before. Most of the band usually plays harmony, while only one or two groups, or perhaps a soloist, will be playing melody. However, I can't think of a specific term for folks who play harmony and those who play melody. I think if you were to say "lead instrument", musicians would think of the top player in a section. I don't know about "backing instrument". Some artists do hire what's known as a backup band (a group of professional musicians) to record with them on an album or play behind them for a concert.