I suspect that Amy's finding of the ring is the critical part of the last episode. I think that Amy is going to remember Rory, and that River Song is either going to help her, or teach her how to fly the Tardis. At that point I think that Amy will take the Tardis, cross her own Time stream to try to rescue Rory and, in doing so, cause the explosion that starts the cracks.

How they get out of it, however, I do not know.

Also, as a small point. How would the Doctor without a TARDIS get around time to repair things? Jack's old timetravelling macguffin was destroyed, was it not, and I wouldn't see Moffat using one of RTD's old standbys anyway. So how, do we propose, could a Doctor travel time without a TARDIS?