1. Boss. He plays Mario and Luigi, and is currently the best Luigi out there, probably.
2. It depends a few factors. Weight and fall speed, meaning Falcon is nearly helpless against it while Jigglypuff is pretty much immune to it, DI, obviously you want to DI out and away from Mario, and in some cases, the frame of their aerials. I can't give you a list myself, but I can give you a frame chart.
http://pastebin.com/NJ4Q5PzU
3. Up throw is almost always a better option over down throw at lower percents. Up throw doesn't just combo into Tornado, it can combo into up air strings, back air, and even Super Jump Punch sometimes. I'd recommend using it on a heavyweight like Bowser, because doing down throw combos on heavyweights will only get you punished.
4. To tell you the truth, you might just have to play around with it and find out. Mario can be very creative with his combos. Get a feel for them.