Hi @eric.tx,
I have created a new post asking for your help to integrate the pulseaudio echo canceller.
On the other hand, we have been testing and we believe that the echo is not due to acoustic feedback but to a failure of the WM8904. It seems to have a very high crosstalk between the HP and MIC signal path. @eric.tx have you tested if with another codec you get the same echo? Have you tested with the NAU88C22?
Julián