How to analyse the circuit performance of an electret microphone
my question is really all about how to work with the signal from the
  electret going into "whatever comes next"


You've done the math correctly, -44 dB V/Pa is 6.31 mV RMS per Pa (RMS) (at 1 kHz) and, if you look at the test circuit on page 1, this figure is with a 2k2 bias resistor in place but otherwise feeding an open circuit. The output impedance of the device is specified as 2k2 but, I suspect this is the external 2k2 resistor - the internal JFET (more than likely) amplifier will have an output impedance largely dictated by the external resistor.

Q1 is not going to have either a linear input impedance or one that is particularly high. This is due to the way Q1 is biased so it's hard to take guesses. How much this will attenuate the microphone signal (given it has a 2k2 output impedance) is tricky but if all else fails get a copy of LTSpice and set up the circuit. LTSpice is free and although it has a steep learning curve (like all simulators) it would be useful for you in the future.