We neither know nor care.
Now first of all, I have seen the film, but am still going to call this out as simply physically incorrect. But before writing the answer off as another one of those "the science of the movie was just inaccurate" answers, please let me reason why I still think this to be relevant for your employed viewpoint here.
Now first of all, it has been previously pointed out, that the movie was not as scientifically accurate in many details as a layman might have guessed from a short glimpse. Especially the relative orbital locations of all the objects were entirely inaccurate. The Hubble Telescope and the ISS would have been in entirely different and unreachable orbits.
You said the existing technology wouldn't be able to travel the distance between space stations. How far apart are the Hubble and the International Space Station, for example?
That was the other kind of glaring thing. The fact is, the Hubble is in its particular orbit, the ISS is in its particular orbit, and then the Chinese space station is in its particular orbit. Without getting too involved in orbital mechanics or the orbital insertion planes of these different satellite systems, they aren't close to one another, ever. So Hubble is in an orbit fairly close to the equator … kind of around the belly band of the earth, but at a very high altitude, around 400 miles above the earth. And then the ISS is at a very high inclination, so it sees from basically the latitude of Kazakhstan to an equivalent latitude in the southern hemisphere … so a much greater swath of Earth. The bottom line is that they’re in very different places, traveling at different altitudes and orbits of the earth, so they're not at fixed distances to one another. They're basically speeding bullets relative to one another. Their orbits cross at varying times throughout the day, and it would be physically impossible to get from one place to the other.
Add to this that the evolution and behaviour of the depicted collision cascade would be completely different, too. It wouldn't fly around in such a linear belt of stuff orbiting the earth in a single direction at a speed perceivable by Stone (add to this that the interval keeps at 90 minutes even though Stone considerably moves location).
What's your experience with space debris? If you're out on a space walk, what are you seeing around you?
Well, first of all, if you see it, it's too late. That was quite implausible, to me, first off, the chain of events that would take down the world's telecommunications. Things don't quite move in that pattern. But that notwithstanding, if there were a cloud of Micro-Meteoroid Orbital Debris, what we call MMOD, it would be coming at tens of thousands of miles an hour. I don't know if you've ever looked at a bullet coming straight at you. Hopefully not, but if you had, you wouldn't appreciate it for very long. And these would be a much tinier, swarmlike pattern of debris coming at perhaps twenty times the speed of sound. And you wouldn't see it at all; it would just rip through everything in its path. So it's kind of a spacewalker's doomsday scenario.
Likewise was the whole scene of the Chinese space station being dragged out of orbit highly dramatized and temporally accelerated compared to reality.
What did you think of the scene where she crash-landed to Earth in the Chinese space station?
It was dramatic, but improbable. You start to interact with the upper reaches of the atmosphere, what we call "entry interface," at 400,000 feet, so that's roughly 80 miles above the Earth’s surface. It appeared that the station was substantially above that in the scene, and it probably wouldn't have decayed from its orbit that quickly. That was quite dramatized there, when she's kind of holding onto this bucking bronco of the Chinese space station. It would probably take months or years, even if the station had been hit by lots of and lots of debris, to decay down to that altitude where atmospheric drag would take hold. And that's, of course, how the Skylab and all the Russian space stations were brought down: They fired their engines, lowered their altitude to the point where it could interact with the Earth's atmosphere, and they became fireballs prior to falling to earth.
What those things show is, that the very fundament this specific incident is grounded on is already so different from our own physical reality, that we can't make any reasonable derivations about its plausibility anymore and all we're left with is to accept it as part of the already low scientific accuracy of the movie-universe with equally less in-depth explanation as the other scientific inaccuracies have got.
Now you stated that you're not so much concerned about the scientific accuracy of the movie and this is a valid and fine approach (I for myself don't care that much about it either and enjoyed the movie for what it is, not a documentary). But this doesn't fit to you reasoning scientifically about the plausibility of that scene. For example you use the orbital locations of things (even upto actual numbers) and the structure of the debris wave although we actually don't know much about those things from the movie and the facts we know from reality obviously don't apply.
Stone had to utilise a launcher pod to bring herself down to this altitude, so there obviously isn't enough gravitational pull to de-orbit the wrecked ISS, let alone the tiny pieces of shrapnel travelling at a much greater velocity...
Are you sure she did go down in altitude and not just move inside the orbital plane where supposedly all of the relevant machines are located in the movie? The fact is, at no point does the movie say that the Tiangong and thus the orbital plane that Stone works in throughout the movie, isn't very close to the earth's atmosphere. Neither does the movie say how "high" that debris cloud actually spans and that it couldn't get dragged into earth's atmosphere either.
The debris following the space station is just another aspect of the scientifically inaccurate orbital mechanincs and debris behaviour applied in the movie in the first place. You say you're not discussing the scientific accuracy of the film but the plot flaw inherent in this scene. But I on the other hand would say there is no plot flaw in this regard. This scene fitted perfectly into the plot at this point and neither did it contradict anything established earlier. You say further that movies set their own rules and it's only a goof when they break those rules, but where there is no apparent clear rule set coherent with our physical reality established about orbital mechanics and debris waves, there's nothing to break either in this regard. In the same way you could ask why the ISS or the Tiangong were actually intact at all, given that the debris wave has already passed them at least once when Stone arrives there. But once the movie reality has sufficiently deviated from ours, as Gravity's apparently has, there is no way to scientifically reason inside of it anymore.
So it is neither possibility A nor B. It isn't accurate to actual physics, but it wasn't explained in the movie either, as well as many of the other physical implausibilities. It isn't a plot flaw when seen in context of the rest of the movie but just a physical inaccuracy, which you chose to ignore, though, so other than that I don't see another answer possible here.
