Generally speaking, it would seem that most people use mtn type pedal. My guess is because the cleats do allow you to walk a bit easier (they are smaller and are recessed in mtn shoes). Road shoes typically have a larger platform and are one-sided. This is an advantage on a road bike to add to cornering clearance.
So anyway, to answer your questions
1. People seem to argue the merits of Crank Brothers Egg Beaters and Time ATACs. They are both good.
2. No cycling shoes really let you walk well. Mtn shoes are better than road shoes.
3. Float is the amount you can move laterally before you unclip.