Yep - just let the ground float on the mic side - audio out iPhone to tip - of works just fine. No hum - headphone level full on phone and vox at 3, RX audio from radio 4 clicks above zero. Turn off power saver. I am sending and receiving on 144.39
Did you try simply not connecting the ground at the mic jack of the radio? If that doesn't work, how about a cap?
Thanks - i am just about to embark on the same project as soon as the radio shows up.