I'm not a fan of the shared mix either. If I was in that situation I'd have a little mixer on my board and run an out from the Helix to it directly - take the shared mix into another channel - and have my IEMs plugged into the mixer. At least that way you'd always have control over your guitar level in the mix. I did that before going to IEMs - had a mix from the board into Input 1 of my FRFR (Alesis 112) and my guitar direct into Input 2. Regardless of what I was getting from the board I could always mix in guitar.