For the first part of your question - I think diffusion (or is it dispersion?) - at least at lower frequencies - would allow some of the sound to propogate to someone outside the room.
Sound Waves are pressure waves - and when this pressure is no longer contained by the 4 walls it will push outside the bounds of the walls - when the pressure front moves outside the walls, the surrounding air will react in all directions - including down.
I suppose you mean ADEQUATE rather than Maximum - you can't have Maximum if you are less than all you can have. Then the question is how do you define adequate?... as heavy-duty as the side walls and floor/ground in order to achieve maximum soundproofing?