I don't have any experience with the Ebay mic you asked about... I have had good results with the built-in mic on the X10 units. Outdoors, it's a little tricky, as the wind noise is much louder than any usable audio, other than yelling or car engine sounds. In short, I skip the outdoor audio.
As far as which wire goes where, you seem correct. I have had times where my video didn't look right, and so I switched the wires and it was crystal clear... I swore it should be the other way, and it was much better switched... So I will let you try it. The only thing I can say is that there will be no mistaking it when you hook it up. If it's wrong, it will not look/sound right. I never used a balun, I just terminated the wires right into an RCA or BNC plug, with good results. I never really got the lesson on 50 ohm vs 75 ohm, etc... I could never see the difference in short cable runs. I remember people stressing in video applications, but I generally use whatever I have lying around and have had acceptable results. You would think you'd need 75 ohm shielded cable from the camera to the DVR - but most people, myself included, use cat 3, satin phone cord, twisted pair, or whatever we have laying around... If the video looks bad, we bash X10!
Good luck - and let us know how it turns out!