+1 on fit.

You should be able to get several different brands of nylon soled shoes for $125 retail, less than that if you shop at discounters or look for gently used shoes.

I seriously doubt that the difference in stiffness between nylon and carbon is enough to affect the health of your feet one way or the other.

What did your fitter have to say about your PF? Are you pedaling with your feet plantar flexed (toes pointed)?