A dedicated quantizer or a sequencer with built in quantization will help keep things in tune. Ornament and crime is great value with a quad quantizer built in, scales is a dual quantizer, but you have less control over the notes available to each channel. When paired with a matrix mixer and/or an a*b+c polarizer you get a lot playable improvisation. Now this helps with the no wrong notes, controlling dynamics and when voices play is a topic in and of itself and the options here are many, mutes are a easy cheap option to switch on and off voices during live performance.