There are a lot of questions in this post, but I'll have a stab at some of it.
Firstly, you should read about the difference between MIDI and audio, since from the looks of it you seem to refer to playing sounds through your MIDI controller, and sending those sounds to the MPC. Dig around on
www.tweakheadz.com for some articles about MIDI and audio, then you'll probably start to think differently about things and might be able to work it all out more easily.
From what I read, it seems that you want to use Pro Tools for audio recording, and the MPC for your sequencing, is that right? If it's the case, then what you should do is hook the audio outputs of your MPC and the Roland to the mixer inputs. Take one set of mixer outs and attach these to the inputs of your PC's soundcard. (If you have a soundcard with enough audio inputs so that you can plug the MPC and XV in there directly, you might want to forego the mixer). Take another set of the mixer outs and attach these to your speakers (through an amp if necessary). Or your headphones. This should take care of the audio side of things. Note that there is probably more than one 'right' way to do this, so don't be surprised if you get other suggestions that differ.
In order to get your MIDI side of things going, connect the MIDI out of your controller keyboard to a MIDI in of the MPC. Connect a MIDI out of the MPC to the MIDI in of the XV. Chapter 9 of the MPC 1000 owner's manual explains things from here on, in the section "Connecting the MPC 1000 to the MIDI keyboard with the sound module". The only difference here is that your sound module is not built into your controller, but to the rack. The thing you'll want to take notice of here is the 'soft thru' settings. You should also read 'MIDI sequencer features' in chapter 3, so that you can understand how to record the MIDI data from the controller keyboard.
Finally, the assignable mix outs are so that you can send different sounds out of the MPC to different channels of a mixer, or to different inputs to your PC. Basically, it means that you can send different sounds to different outputs and then apply different eq, fx and so on to each output. For example, you might wish to eq your hi hats differently to your snares, differently to your kicks. So you could send the hats to output 1, the snares to 2, kicks to 3 and so on.
