For example; in the comic book story, "JLApe" (which is what partly inspired this episode), it was revealed that Grodd can absorb the bio-energies of his fellow simians. So, if Grodd's masterplan--in the episode--had continued uninterrupted, the devolved Legion of Doom might have suddenly found themselves dying off. And, a result, Grodd might have also wound absorbing the collective powers and abilities of his "team-mates!"
As for Tala's reference to Grodd letting her out of that "magic mirror?" Considering the episode that preceded this one, I can only assume that he recruited Mirror Master's help. Perhaps, as the price for helping the Rogues' Gallery escape from prison, so they could spoil the Flash Museum's opening ceremonies.
I also liked Mark Hamill being cast as the voice of the Trickster. He originally portrayed the character on the 1990 live-action CBS series about the Flash. And, the laugh he used there is what led to him being used for the Joker's voice on the animated Bat-series starring Kevin Conroy!
Guess it's just a classic case of things coming full circle.