Note that I have not yet hooked up control line B, because I haven't finished wiring up the board yet. Control lines C and D, and in/out C and D are not currently used in the project.
I would try putting a pull down resistor on the output side of this switch to short out any leakage. Something between 1K and 10K should be good enough if that is indeed the problem.
You can't leave the inputs floating, you have to connect them to ground if there is no other form of input.
between the negative side of the circuit and the ground bus.
Here, you're talking about the 4066's control inputs, right?
Yes the pin 2 of the welder signal and ground. I assume you also have the ground of your welder connected to the ground of the arduino, if not you need to do that as well.
I suppose I could check for voltage between the pin 2 (the - pin of the "detect" circuit) and pin 3 (the - pin of the pot circuit).
If I was to use an opto-isolated, solid state relay, would these issues be circumvented?
Pin 4 is connected to a digital output pin that is putting out a voltage between 0 and pin-5 Vcc