Okay I signed up on this forum just to jump into this conversation.
First, Absolute Zero is when all heat is completely removed from matter. All kinetic energy is gone meaning all known forms of energy may be gone. The problem with that is basically it does not disclude the idea (in a Whovian world) of life still existing. The Cold star presented in the recent 11th Doctor episodes should easily prove this point. It just proves the idea of energy being able to exist on an opposite scale that uses cold where heat is used in everything we know. So just because the pocket universe attains absolute zero it does not mean everything stops. Just all normal heat infused matter as we know ceases to exist.
Second, we may be looking at the pocket universe in a bit of a human way. The pocket universe may have different physical laws which is why the Doctor tests for gravity and such upon arriving to make sure the basic laws that Amy and Rory know of still apply so that they can survive. And if the laws of physics there are different then it is totally possible that time does not travel normally there in which case the TARDIS may not be able to travel in time the same way. It is conceivable to think that Sexy was stolen from the TARDIS before she had an opportunity to calibrate herself to the type of time flow that exists within the pocket universe. Hence the term Time And RELATIVE Dimension In Space.
However, assuming the TARDIS could calibrate to the pocket universe's flow of time it stands to reason once again that since the universe they know has a causal nexus because of the time continuum that any pocket universe that exists would also have one in relation to it's own flow of time. This easily means that just because the TARDIS could travel back in time to meet the Corsair it does not mean it is a good idea because this could throw the causal nexus into a state of disarray. The example of this is when Capt Jack makes his way back to earth with his Vortex Manipulator. He had to wait a few hundred years for a version of the Doctor to arrive that would correspond with a version of himself, otherwise he could create a tear in the time stream causing past and future events to leak upon the present. This is also why Captain Jack "visited" Rose a few times while she grew up, during his time on earth, but never interacted with her for the same reason.
There is also a reason why the Doctor does not cross his own timeline. Remember time in general in the Whovian mythology is non-linear. This means that it is a bit like imagining a cube (3 dimensional) within a sphere (the outside of the sphere being a higher dimension than time and inside the sphere being everything that must follow the rules of time)...which brings about the idea of wibbly wobbly. The Doctor does not intentionally cross his own timeline unless it naturally presents itself. Think of it this way time is an environment similar to the rules of Mother Nature or Gravity. Sometimes these systems can break their own rules for whatever purpose. So time can also lead the Doctor to cross his own timeline creating a naturally resolving paradox...(like a roller coaster that loops around itself and continuing on) the specific paradox is linear and unchanging but if you look at it from an outside vantage point it a naturally occurring phenomena. The reason the Doctor mostly does not ACTIVELY usually choose to cross his own timeline is because this could possibly create an irresolvable paradox which could create that pocket temporal dimension, in which those creatures come to eat all the time within to destroy the paradox from existence as a whole. But when he crosses his own timeline naturally (for example mini episode Time Crash) it becomes a linear event in which case each timeline bisects the other and continues on into the wibbly wobbly version of time. There are opportunities (as he can see all of time and space) in which case he can use the causal nexus, as it exists in our universe and our flow of time, to completely change the state of a future (or someone else's past in some cases apparently) but usually the action has to lead to the same overall result so it does not domino effect the "fixed points in time and space" in any adverse manner. Keep in mind he can try to effect the fixed points all he wants (The Waters of Mars) but as of yet there is no way they can be changed because (theoretical statement) maybe these events hold such a weight with the universe that if they were to change it would completely eradicate the causal nexus since so many future events would change because of it, essentially causing this dimension to crash into many other dimensions (dimensions being the ones that are separated by the Void...remember Voidstuff episodes?).
On to the next response. (Phew this is fun) The time war IS time locked but not all events leading in and out of it are. Once again only major events leading into it are. It is easy to guess that there are minor events that had no real bearing on it that are locked which is why that Dalek had an emergency temporal shift that helped him fly right into the heart of the time war because he found a small tributary of time that had no significant bearing on the time war which transported him in. So things like the creation of the Timelord Houses would be locked off where as the disappearances of the Cult of Skaro in the very beginnings would not have mattered considering they were a brain trust not any sort of active soldiers raining down doom in the Time War. You have to remember that although the Time War is a horrific event the Doctor does not lock it off because of the horror involved but because of the Terror the Timelords are about to inflict on the universe. They were going to become beings of pure energy and in doing so destroy 2/3 of this universe (and maybe multiple other universe's who knows) in the process, effectively wiping out trillions or quadrillions or quintillions of lives for their own purposes of escaping a war that brings thousands of timelords back to life and kills them again hundreds of times to test out thousands of temporal variations of the war. So the only truly necessary parts of the war which would have to be absolutely locked is anything involving a Timelord. He probably locks most of the rest off because he, being the benevolent Lonely God he is, wanted to spare the rest of the lower species the horror of multiple deaths because of the consequences of higher species inflicting pain on one another. The time lock also cannot effectively erase any species from history it does only as it states which is locks it off. This explains why the lock was being penetrated by The Master because he was a link with the Timelords while they were locked away. If the Time Lock DID erase them then that Wooden woman in Rose's first trip would not have known what a Timelord was and no one else in the universe would either.
Lastly, I think, the existence of more pocket universes is DEFINITELY a possibility. I would say there could be an almost infinite amount considering you cannot be limited in the number of possible choices being made at any given point in this dimensional variation of the universe that could create one, but the number is most certainly finite. It would have to be finite because the theory is that the Universe is an ever expanding and contracting big bang. This means that you would have to be traveling faster than the big bang to create a pocket universe because you would have to be able to escape into nothingness to do so. The number of sentient creatures in the universe being very large are still however a limited amount, which means the number with the capabilities to create a pocket universe would be very very limited as well.
I know this is tl & dr but meh. Have fun battling over all this.
