RC filter for emitter follower input
My question then, is why is R determined as the parallel combination
  of R1, R2, and the transistor input impedance? Or indeed, more simply,
  why is the input impedance of the voltage divider R1 and R2 equal to
  their resistance in parallel? -They are not in parallel, they are in
  series between +15v and 0v.


You need to think like an AC signal - an AC signal doesn't recognize +15 volts as anything any more special than 0 volts. It regards them both as an infinite AC current sink and therefore, for the AC input signal, they can be regarded as both connected to 0 volts or, both in parallel to ground (or whatever terms you want to use). Ditto the resistive impedance offered by the base.