I don't think that should worry you too much, there's no balanced connection from the mixer, but that's probably not the "weak link".
also, as far as I know, line level signals (like the control room output) are less vulnerable to noise than long cables with microphone or instrument levels.

I think you'll see that the noise from interference from short unbalanced lines is less than the noise from the mixer itself,
unless you have some very bad interference, and then you should anyway take care of it.

anyway, a possible sollution is a balanced trim component between the computer and the monitors.
there are boxes made especialy for this (signal routing and level control).