RE: Diamond Dust + 2.4GHz
Doing a range check like that (walking around with the plane with the engine running) is the best way to do it as far as I am concerned.
As far as the issues you described with the rf black hole you have going at your field have you tried using a toroid coil on your servo and power leads to the RX? I had similar issues with nearby sources of RF myself and that resolved my problems, granted it's not a magic bullet for all rf issues but helps in some cases.
As far as the DD I have seen some pictures of them with CF Spars and LE's but can't have not seen a lot of them other than in pictures.
I have three planes with 2.4 Ghz setups in them two that have the antennas are internal (classic balsa and film types) and one with carbon fiber and external antennas. It is my understanding that as long as your plane is not covered in a metallic film or carbon fiber having the antenna internally with 2.4 Ghz is not a problem. The more metal or conductive materials used in your the more the possibility of radio issues with internal antennas is my understanding. A few years ago when I first got into a setup with carbon fiber and a 2.4 radio I asked a question about it in a couple of forums, and what I have related is pretty much what I have been told. There is one interesting bit of information I did pick up a few years back when I first asked about it, a few guys with the dual receiver JR radios had the antennas internally on their planes and said they did not have any problems with their carbon fiber sailplanes.