...If there is a possibility that the wires pick up noise then no wire should be connected to the circuit directly. ...
The rest of the circuit is really normal and simple stuff.

..The original description says he doesn't care how much the transition is delayed/shifted in time. I'd guess it will work fine, and deosn't need an optoisolator at all.

As I suggested, if he takes those 12-volt inputs from the relays and divides them through, say, a series pair of 1300 and 820 ohms, he'll have, nominally, his 2K-ohm resistance from 12 volts to GND, yet his Schmidt-trigger will see a transition from about 4.8 volts to GND, and if he puts a 1 nF cap across the 820 ohm resistor, the risetime will be slow and relatively bounce free. The Schmidt-trigger will swallow (mask) minor (100 mV) noise or ripple on the input, since its threshold for positive-going signal is considerably higher than its negative-going threshold, in fact, nearly a volt. The threshold doesn't move around much, as it's ratiometric (in the case of the 40106) and tied to the supply.

The original description says he doesn't care how much the transition is delayed/shifted in time. I'd guess it will work fine, and deosn't need an optoisolator at all.

If you properly filter your power supplies you needn't worry about RF or AC from the mains with the low impedance that you're using.

Unless the ambient noise is quite low-impedance, you shouldn't see it in the circuit you're using, particularly if you capacitor-damp the schmidt-triggered input.

The problem is you are putting an analog signal into a digital input.

I would like to omit the optoisolator.