There actually is a very subtle clue that Matt's return towards the end was just a dream. Considering how he was usually depicted as being extremely professional, it does seem a bit strange that the real Matt would just open the door to Ryan's capsule and expose her to the vacuum of space without at least giving her a moment to put on her helmet.
Recall how the Tiangong was reentering the atmosphere? Given how the orbits of all the stations were in the field of the debris field, what if it was intentionally deorbited by the Chinese taikonauts before their departure in a deliberate attempt to prevent it from contributing to a potential Kessler Syndrome?
Remember how Planetes made such a big deal of the horror of a Kessler Syndrome, which can potentially isolate Earth from space? More than likely, it's coming to pass by the end of the film.
A more realistic scenario would have been even worse then that depicted in the movie. The expanding Kessler storm would likely not have reached the ISS, shuttle or Tiangong (since they all were in higher orbits), but would have made it impossible for any of their crew members to reenter without being shredded by it. They would have been trapped in space to slowly die, out of reach of all rescue. That all by itself would have made a good (but very different) movie.
Well no. As stated in the accompanying documentary on the DVD; it's easier to cross the road than live in the road. Would be tricky but perfectly doable to get through the debris.
The debris field would still largely be in one (gradually expanding) cloud, as they're mostly moving in the same orbit as the satellite they originated from. This means getting past it is just a matter of extrapolating the old satellite's orbit and seeing where the cloud will roughly be. This is especially easy considering that the Gravity scenario relies on an oblique set of orbits to be feasible, as objects in the same orbit always move at the same speed (thus it not being possible to "catch up" to an object in the same orbit without power). This means any crew on a higher orbit just needs to pick a descent profile that cleanly avoids the debris field, without needing to perform a plane change. This only lasts a short time, though, until the debris spread out enough to a more even and thick ring. Any collisions with that would then potentially cause what we know as Kessler Syndrome.
If Stone had rested aboard the ISS for just a minute longer, she would have been caught in the fire, which engulfed the hatch to the Soyuz a fraction of a second after she went through. And if the fire hadn't forced her to evacuate when she did, she wouldn't have had time to flee before the debris cloud's second pass. Finally, if there had been no fire at all, she wouldn't have needed the fire extinguisher that proved even more vital getting her to the Tiangong than actually putting out fires.
It was said that the initial impact event which triggered the debris storm was a Russian attempt at shooting down one of their old military satellites. Even if the missile strike was a misguided attempt at de-orbiting a dangerous old satellite, think about how the White House and the Pentagon are going to see this. A Russian missile attack basically slaughtered (presumably) seven (they don't know about Kowalski or Stone) US astronauts, obliterated the Shuttle Explorer; and wiped out the entirety of American GPS navigation satellites, secure military communications satellites, military and civilian strategic photoreconnaissance satellites, and nuclear missile launch warning satellites. That's a borderline act of war. I don't think there's any President of the United States imaginable who would hesitate for a moment in sending the US Armed Forces to a global state of DEFCON 2 readiness, with instructions to standby and prepare to go to DEFCON 1. Hell, if I were POTUS I'd be boarding Air Force One to get the hell out of D.C., as well as making sure the US Navy's E6-B "Looking Glass" mission is airborne to serve as a backup nuclear command post if Air Force One was shot down. A massive surprise communications and surveillance blackout like that would be the perfect time for America's enemies to attack, so I think it'd be logical for any hypothetical POTUS in this scenario to be Properly Paranoid. As Stone is parachuting back to Earth in her salvaged Chinese Shenzhou capsule, the world stands on the brink of World War Three and Mutually Assured Destruction.
