I've made a duplex box myself with a 5 pin and have had no issues. I tied both grounds together on the 5 pin. I've been using it for over a year without fail. Need to sit down at some point and make some more boxes and a few extension cables.
I have a pair of ie-10s that had the cable replaced before I bought them. From what I understand the Ultimate Ears universal accessories should work with them. http://www.logitech.com/en-us/ue/ue-earphones/accessories