The Apple techs statement that you cannot use an Airport Express to extend a non-Airport network is news to me. I have been unable to verify that independently but that does not mean it isn't true. However I will take a shot at explaining what you are seeing in AirRadar

  1. Signal is the instaneous signal strength seen on the last scan
  2. Signal Avg is the average signal strength over all the last n seconds. The scan interval and retention in seconds is configurable in AirRadar > Preferences > General
  3. Signal Max is the peak signal strength seen in all the scans in the last n seconds
  4. In both rooms you should be seeing readings for your base station and your repeater but I would expect them to have the same SSID and to be operating on the same channel. (Bear in mind I am using a Time Capsule and Airport Express so your results may be different

If you are seeing a significant variance between Signal and Signal Avg and have a low noise number I would suspect something on someone moving around in the space and interrupting the signal line of site between your computer and the network device you are connected to. An intervening human or animal body will attenuate a signal just as a wall will or heavy furniture will. Perhaps not as effectively as a sheet of lead or grounded melt wire screen but it will cause attenuation.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein