One has to wonder why Krang left the portal open for so long. That just seems like it's encouraging the Turtles to come back through as well. Although, maybe he didn't realized the Turtles were there. That's possible, and a decent explanation.
After I set this up, I realized that the other two Shredders were still around. In the game, they were only temporary, fake clones, but here it seemed funny to keep them around. In the main arc, they actually still exist and are denizens of Veros.
 
    

When evil spreads across the land, and darkness rises and the monsters roam. When the creatures of the night make beautiful music, and the things that go bump in the night go bump with greater enthusiasm. When the world is in peril and is in need of a hero...

These guys are, sadly, the best the world can hope for. These are the adventures of the heroes of CVRPG. They mean well, they try hard, and occasionally they do the impossible...

They actually do something heroic.