[magno]

What you could use is a small mixer in your booth, something with say at least 4 inputs. Now set up some aux groups or sends with whatever the artist needs to hear(in your DAW). Of course you are limited to 2 outputs with your interface now, but theoretically you could send out a mono mix of the instruments on one channel and say the vocals on the other one. So your singer can control at least that much. You may have less issues with latency if you decide to get an audio interface that has routing features built in to it's mixer software, but you will still need some kind of mixer in your booth.