I also would use the AD8476.
While you are correct that the microphone output impedance will reduce the otherwise perfect performance of the differential amplifier, it will not matter in the big picture. This is because the low output impedance and the twisted
pair cable will ensure that any possible 60Hz pickup will be unmeasurable.
You may want to drive your twisted pair via some small resistors and you may want to use a common mode filter on the receiving end of the twisted pair, because high common mode rejection on the receiving end is only for low frequencies. Higher frequency electromagnetic pickup is not necessarily removed.