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