Op amp circuit and input impedance
It’s a Schmitt trigger circuit. All that gobbledygook on the front end is there to protect the input, and to bias the op-amp (actually a comparator) inputs at a working midpoint.
The sensor signal is AC coupled to the (-) input through C786. It's biased at 3/5 VCC by R305/306, to a net impedance of 120K. R307 acts like a voltage divider with R305/306 and reduces the gain a bit, acting as a high-pass RC filter with C786. The actual input impedance is frequency-dependent and is about 195K at the right side of C786.
Going further up the input, R281/R318 and D515 form a biasing network. D515 is reverse polarity / leakage protection.  Overall impedance will be a bit less than the divider you add in the downstream stuff: about 1K depending on frequency.
R308 and Z1 are input protection. Z1 limits the voltage swing to +5.1 / -0.6V. R308 limits the current to Z1 so it doesn't take the full brunt of an ESD spike.
D516/D517 further limit the input to VCC +0.3V and -0.3V. Being Schottky types they have faster response than the Zener Z1. Kind of belt-and-suspenders in my opinion.
On the comparator (+) side, there's the biasing network R282/283. Not only does it set the 3/5 VCC operating point, it works with R309 to make the hysteresis snap-action (Schmitt triggering) to square up the signal.
That all said, if the designer were that concerned about transients, they haven't taken care of the ground input. That's a major vulnerability. I would add a common-mode filter between X1 and rest of the signals.