1) the Picade HAT is a self-contained product but only support a single channel / mono speaker. Volume up and down is supported out of the box via bcm 13 and 26, on the right hand side, if you want to make use of those, though they can be repurposed.
2) the power functionality is tied to a specific GPIO, bcm 17, at least in term of the software behind it, though technically it could be customised. Still, there is a dedicated connection on the left of the HAT.
3) it is really up to you. If you change the key bindings in the Picade PCB firmware you can double your number of inputs, and only use a single joysticks... for a LOT of buttons.