Electronic – Will standing on a non-conductive surface with wet feet prevent grounding and therefore electrocution

(1) Does 'input 2' correspond to the +12V wire and 'input 1' to the -12V wire?

NO. This is connected to the power supply connector. (Pin 1 is the positive, pin 2 the ground or 0V). The INPUT connector is where the audio signal goes (2) is the live and (1) is the ground.

(2) When I see the symbol 'ground' on the schematic does that mean that I need to connect those to the -12V line? I have no 0 volt line in the circuit so what is the ground line?

All voltages are relative to each other. If you only have TWO wires from the AC/DC converter then the most positive is taken as the positive and the other one is the OV or ground. A simple check with a voltmeter will determine which is which.

(3) Why are C3, C5 non-polarized capacitors and why are the other ones polarized? The current flows from negative to positive. Does that mean that the polarized capacitors need to be placed with their positive legs pointing to the left?

C3 and C5 are small value capacitors (0.1uF). These can be easily made as physically small, non-electrolytic types. They have the advantage of being able to decouple (short out) the higher frequencies. The larger values (uFs) are made as electrolytics as these can be made with high values in small physical packages. They are generally much poorer at handling the high frequencies. By combining an electrolytic with a non-electrolytic capacitor in parallel (eg C3, C6) you get a much better response over a wider range of frequencies. In this case they are used for 'smoothing' the supply voltage, preventing hum and hiss. The positive plate of an electrolytic is shown as an open rectangle but left and right (or up, down) have no meaning in terms of connection as this will be determined by board layout. Conventionally current is taken to flow positive to negative.

I don't like (2) very much from a CAD perspective because then you can't have separate nets for AGND and DGND.

In Altium you can use a component called a "net tie" to physically connect two nets in a layout without actually merging the two nets into one. Other CAD tools may provide similar capabilities.

Is there some reason not to go with (1), in terms of noise performance?

Your option (1) forces the return currents for the digital signals to find a long path back from load to source. This means the complete signalling circuit will encompass a relatively large loop, which can result in coupling to other circuit loops and/or emissions.

Is there a better way to keep the digital noise away from my precious analog circuits?

Your option (2) is one way.

You could also provide a capacitive path between the two ground nets near where the digital signals cross. However this can still cause some crosstalk between the digital signals due to the shared constricted return path.

You could also use an isolating device such as an optocoupler or transformer to carry each digital signal across the gap between the two ground planes. Normally, due to cost, you'd only use this technique if you need to allow for substantial voltage difference between the two ground nets.