Simple but game changer would be to cut the monitoring in the cue when punching a track, this way no need to have a direct monitoring sound card to have it working. From the singer point of you, he ears himself thru the sound card mixer directly where it is mixed with his cue. From the engineer point of views you ear first the sound of the track and when you punch you ear the input sound (with latency) while it mute the track in the singer cue the time you are in punch in. This must be possible to do I suppose ?