Mixing all sound inputs in one page for your Headphone output, Instead of mixing aux signal into your guit mix. [See the attached image for an example]
This will make it very easy and more flexibel to create your perfect IEM mix on the fly.
Return Input = Aux in L
Alternative Input = Aux in R
Default settings will be for a stereo Aux in, when selecting Aux in Mono, Aux in L = summed to headphone L+R out and Aux in R = Also summed to headphone L+R out, so you can mix 3 different sources to your headphone.
Also, if the monitor mixer engineer can't give you a mix without your guitar signal (because you must share the same monitor mix with other musicians) , you can tun your (internal) guitar volume to MUTE to prevent phase issues.