I'll give this a shot ....

Question #1: Yes, you can use this thing live.  It's a MIDI controller so it has no internal sounds of its' own.  You NEED a computer to use it, period.  No computer, no workie.

Question #2: Yes, depending upon what software you have in the aforementioned computer, and how you have it set up.  I use Logic, Mainstage, Reason, Ableton, Sampletank, a hardware synth module and a host of AU's, VST's & whatnot, and I do have it playing multiple sounds at the same time, so it is possible, I also have a mic plugged in as well for vocoder-type sounds, and I use it for guitar sounds too.

Question #3: Yes, you can control any parameter with footswitches and expression pedals.  I use a Boss FS-6 Dual Footswitch and a Line 6 expression pedal.  Make sure your MIDI CC information is mapped correctly, and you might need to use a librarian to control everything, or be real handy with your laptop.  It's kind of a pain to set up, but it does work once you get it down.  If you need more control than that, a MIDI USB footcontroller will work, provided it has the correct drivers for the software.  The Line 6 FBV Shortboard will work with it, and I've seen somebody use a MIDI Buddy with a KB37 as well.

Question #4: Yes, you can set up different levels utilizing the mixer in Pod Farm.  It will remember the different levels, and there's always the global master vollume available at all times.

Remember, this is a studio tool, so using it live isn't exactly what it was designef for.  If you're looking for extensive real time control, you might want to consider a master MIDI keyboard like Pro Keys 88 or something like that which would offer more control over MIDI mapping and automation.  All of what you'd described is possible with the KB37, and I've done it, but it requires some work to make it happen.  The internal mixer in Pod Farm only allows for two channels of sound from external sources, but the keyboard sounds are only limited by your DAW software in your laptop.  You will probably need a dedicated librarian/automation suite like I'd mentioned before to do all this stuff on the fly, that's the only caveat I'd look out for, but this is a great piece of equipment and is very flexible.

Good luck!

Namaste