Universal Analog Input
For this low-accuracy circuit, I suggest two n-channel logic-level MOSFETs, one between R14 and ground and one between R12 and ground. The first one needs low Rds(on), the second needs reasonable Rds(on) in relation to R12 (a few ohms is okay) and low leakage so not too much voltage is dropped across R11 when open.

I would suggest adding resistance before the zener to limit the current and adding a resistor (perhaps 100K) from input to ground so that the input is not floating with both MOSFETs off (which would drive the outputs high, causing the LM324 to try to dissipate ~2W for 4 channels used, and each zener over 200mW. The LM324 datasheet says: 


  At values of supply voltage in excess of 15 V,
  continuous short-circuits can exceed the power dissipation ratings and cause eventual destruction. Destructive dissipation can result
  from simultaneous shorts on all amplifiers.


So even a single channel shorted is too many with your 18V supply.