Despite the fact that the ending was predictable the moment the stasis cube tech was introduced/explained, I thoroughly enjoyed this special.

I agree with Mix in his reply above about the Time Lock. It was the Time War that was "locked", not the planet Gallifrey itself. So even with all the timey-wimey shenanigans going on, it's still possible to manipulate the moment of Gallifrey's seeming destruction. As long as the key events in time were still allowed to play out, the timeline will not be compromised. (ie destruction of the Daleks, the "elimination" of Gallifrey from the picture, High Council/Rassilon's failed plan, etc).

The one glaring flaw in the episode, IMO, was the whole sonic screwdriver calculations running for hundreds of years nonsense. Unless these calculations were either backed up onto the TARDIS memory drives and/or is linked with the TARDIS thereby running the calculations along with blasted thing, I don't see how it's possible the current Doctor was able to enact the stasis plan in the first place! Why? Because the sonic screwdriver was lost or destroyed and replaced many times now!