Some good advice here already!
You could try passive sound isolated over ear cans while wearing the IEMs, and make sure the IEMs are not faulty. The less noise you are exposed to the easier it will be to get a soft but clear signal.
EBSfreak has a good idea using octave up, maybe add a good EQ to cut disruptive frequencies, like the Mxr 10band, or even a rack unit.
Anything that works isn't stupid! I see no downsides to playing live with headphones on.
There must be an audiology professor at a university with experience in this sector, maybe you can find one? I know most academic hospitals here in the Netherlands have someone with a passion for music who know about this sort of thing. (Jan de Laat at Leiden UMC or Alex Hoetink at Utrecht UMC for example).
Best of luck!