on the sub question:

either input or both will give you good results. It is possible that the one marked "mono" may be marked so because its ground is different from the other, or maybe it is just so people who need directions will HAVE one here....

If you connect a single signal through a Y to both inputs, then the two signals will be summed, meaning that they will appear to be a single signal 3 dB hotter than if the signal just went into one input. The difference, then, is that
a)at full volume, you will be able to get 3 dB more out of the sub because you put 3 dB more in,
or
b)at full volume, you will be able to get more distortion out of the subwoofer because one input was just the right level to begin with, thank you,
or
c)you won't ever get it near full volume, you just will turn the knob up slightly less.

Of course, as an installer, you know that you can make some money on a nice gold metal Y connector and a six to eight inch Monster Cable...totally ripping off the customer if you do so, as this is NOT NECESSARY!

and adds no improvement, either