On the rear panel of the MS there is a Power AC IN and, I suppose, a Power AC OUT like any regular computer for the LCD monitor power supply. Do you think I can use that power supply to connect my 2two M-Audio Bx8a monitor speakers (max 130 W each) and the mixer? Can it supply the needed power? I intend to turn on/off the MS and the rest of the equipment together so I simplify using so many switches.

Yes, you can use it, but this power out don't have the switch, the power is wired from power IN to power Out.
when you power OFF the MS, on this connectore you still have the power.
Inside the power supply ATX, on the Power IN connector give also a FUSE protection, not sure if your mixer need a lot of power, BUT if will brocken the FUSE protection, you can change it.

For what you want to do, you need a Relè system switch on the midle.

Ok. Thanks Dom.