The way I saw it Walker was a minor villain after season one and there was no question That Dark Danny was stronger, being one of the mega ghosts.
It might be interesting if the story wrote that Walker becomes more powerful, the more people he keeps imprisoned in the same way Ember becomes more powerful, the more people hear her music. And with the recent apocalypse? there is an increase in the number of ghosts. But I really feel like Dark Danny is weaker in this story than he is in the series.
As for the "apocalypse", I am a little bit confused, and I did not used to be (did I pronounce that right?). Blord, I think, believes it was just Megavill. BR seemed to think that the human race was on the verge of extinction, if not already. But I do not think BR is the writer for GT, at least I do not think he is. The impression I had was that the world had been decimated and only a few safe havens existed. I guess only time will tell as they may return topside or they may not.