For me personally I favour the legwork because I'm light and need to stay mobile more often than trying to stand ground against much heavier people. Sparring with the legs is very dangerous, imho, unless there are set rules (again there's the word!) not to strike certain points but I do feel it is useful to allow a student to 'let it all out' in the safest possible way, so:
1. Yes. Pad up with whatever works (and remove all footwear!)
2. Go as hard as you can with no footwear!
3. Yes. All my legwork is (was lol!) trained through the wooden man and sandbag/kickbags (with no footwear!) and partner shield practice.
4. I would hope that you can find some decent kickers, especially if you're talking about stuff above the waist. But yes, defend AND attack the legs of a kicker always