Here's the first question out of the way: Hideo Kojima doesn't know if Metal Gear Solid V: Ground Zeroes will be released together with The Phantom Pain--so the series' creator told CVG in a recent interview. With that in mind, he at least made the case for the strange division in the upcoming cross-gen game:
"The Phantom Pain provides a vast world where the player can really go anywhere and get lost," Kojima said. "But for past Metal Gear fans used to a linear experience it would probably be hard for them to get used to that without having a prologue and a way to get used to it. So Metal Gear Solid Ground Zeroes is an interpretation to really give them a very short, smaller version of that scale."
Ground Zeroes' is set solely at night, and in a much more constrained region than The Phantom Pain. Playing in that order, series fans can get used to controlling Snake before they begin to tackle sneaking around an open world governed by the cruel march of the sun.
Kojima said he was set on making an open world to allow "users to create their own gameplay." He's being extra-literal there--players will be able to create and publish missions through tablet and smartphone apps and find them scattered around the game.
Kojima Productions' LA Studio is making a new Metal Gear Online multiplayer mode for the game, too, so MGSV should pack a lot of bang for your buck whether Ground Zeroes and The Phantom Pain are sold together or not.