To finish off regarding the input pre-amp/switch:

This is no pre-amp at all. The part turns out to be a dual 4:1 analog mux with a common part number of 74hc4052. The two select inputs permit a single sensor to be selected and the analog forwarded to the knock filter module. With the PRU you can also select both channels at once. In this case the output sums the two inputs. AC input in the standard (left/right) configuration is attenuated to half the input value. Plan to update the schematic, it will remove a bunch of ?s

-Tom