[This would be the follow up.]
With the zombies dealt with Bella pulled out a familiar looking piece of technology and said, "Ziggy says that even though we didn't arrive at the incursion start, what we've done is enough to put the timeline back on track."
"You don't seriously have..."
"No, of course not. I got stuck in a white room inside a no-ship in a black hole for two weeks with nothing but a sonic screwdriver, a temporal delta stream calculator a pile of electronic equipment, and a shipment of props from American TV shows from the 1960s to the 1990s intended for display in a museum in the Glaucon Quadrant. I made some modifications, is all."
"It's not a Quadrant and I have no idea where the name Glaucon comes from. It's not important. The important thing is that we can go back to 2052."
"Why not go the other way?"
"Why not go back to 2012 and solve the problem at its source."
"Ok, first off, the source isn't 2012, it's 2052."
"But it started in-"
"2010, on the platonic ideal loosely attached to a server in Japan, which means trying to solve it there would not only not work, it wouldn't get you home."
"But I could-"
"Furthermore, this isn't our timeline, the changes were so great as to spawn an entirely different timeline disconnected from our own. Meaning that the anomaly didn't grab the you from this timeline, so unless you plan on killing him and taking his place you can't slip back into your life."
"And that's assuming your life even exists in this timeline, which it might not. It didn't just duplicate the past from our own timeline, it organically spawned one by building retroactively backwards the way most timelines build forward meaning that any possible 2012 that might lead to the 2052 we came from could exist in this timeline and not all of them have you in it because there are such things as random chance, free will, the inertia of non-nondescript choice, and the six other axes of causality and given the severe world altering events in the intervening decades there's really no reason to assume that the 2012 in this timeline is all that similar to the 2012 in yours."
"But even so we could-"
"And you're ignoring the fifth dimension The knot in time that started all of this? Well it's going to work itself out sooner or later because nature abhors a non-euclidean tangle of non-temporal causality like that. The time that we might spend in twenty ten or twenty twelve would be time that the knot was spending unknotting itself, and when it finally did that the cross timeline connection would be severed, possibly stranding us here forever, as the time link snapped into this timeline and all of a sudden an army of two billion zombies, give or take, and one evil incorporeal being with untold power over the universe, would suddenly find themselves with a stable time bridge back to the beginning, right where you were camped out.
"And if you think you can fight that off with a katana-"
"It's not a katana, it's a-"
"You're missing the point."
"No, I got the point. We have to go forward to a future that is being killed off by nano-plague and zombie plague, has an active slave trade, is at the mercy of conspiracies, makes libertarians salivate, and has been invaded by the forces of an evil AI who somehow can screw with the real world to stop something or other from happening or else we'll never get home and everyone will die horrible deaths."
"Exactly!" She smiled, "Ready to go?"