well, in general, you should have right, left, and mic, with a ground for each on the iphone end, and all 3 will have a common ground. for the headphones, right and left, with a ground for each, and ground is shared.
you will want to remove the protective coating with a microtorch or other heat source, solder and heatshrink, as opposed to twist together. You will also want to look up the pinout for the iphone side of it, and use a continuity tester to identify what each cable usually is.
red is USUALLY right, blue or green is is usually left - the white on the sennheiser is unusual (i double check this either by following the cable to the end, or using a temporary connection with crocodile clips between a prepared, tinned end of the headphone cable and jack).
Red to red, white to green, and 'copper' to 'copper' should do the trick. all 3 have the pesky coating, so you'll need to remove it.
this is the exact process/tools that have worked for me in the past - but instead of the jack, tin both ends, press to check, then solder.
I expect you are right about what the wires are. You should check with a voltmeter though to make sure. Ground should have the lowest voltage, power the highest, and the return audio somewhere in between although it may be almost the same voltage as the power but at a higher impedance. If you load the audio line with 10 kΩ or so, it should drop a bit whereas the power line should not.
By the way, unless you have specific documentation that says these are condenser microphones, I think they are more likely electret. However, that doeesn't matter for the purpose of extending the cables.
I would use stereo shielded cable to extend each microphone. Obviously the shield should be tied to ground. Since you're only going 8 inches, I don't expect any significant problems with that. Try to leave as little length as possible for the audio wire outside the shield.
Best Answer
The XLR connector is now a standard for balanced microphone connection. In this configuration the microphone capsule is wired to L and R and neither is connected to screen. For an unbalanced microphone one of the mic wires needs to be connected to shield.
simulate this circuit – Schematic created using CircuitLab
Figure 1 and 2 show how to "unbalance" a balanced microphone for an unbalanced input. Figure 3 shows how to connect an unbalanced mic into a balanced input. (Connect one of the L or R pins to shield in the mic plug.)
The circuit assumes a low-Z microphone feeding into a hi-Z amplifier using a transformer for impedance coupling. The transformer is shown for simplicity but can be effected by electronic solutions.