Make sure PP3V3_S4 and PP5V_S4 are making it to the I/O board by measuring J9500, preferably by checking on the I/O board itself. Check all the AP, clk and L signals related to WiFi on J9510, make sure the reset signals aren't stuck on low.
Ensure 100% that you have tried a known working (not taking someone's word for it, make sure these work in another late 2013) I/O board, cable and Wifi card; make sure they are exactly the same versions as the original parts.
TBH, the wifi is pretty solid on the Retina models; they are not like the older models where the ferrite beads on the board tend to burn out. If it is not the parts listed, it is going to be something miserable on the logic board, most likely the PCH.