My guess on earth-2 "2" is that it might be something similar to the saying "Nature abhors a vacuum" since Power Girl and E2 Superman were anomalies that had no hook on the new DC universe which is why there's a Supergirl, Power Girl's alternate self, unlike her teammates the original JSA who are still part of "earth-1" there by making that JSA an alternate version, like the Marvel Family of earth-5. She was a fluke that slipped through the cracks, while E2 Superman, as well as his Lois and Superboy Prime were saved from the wave by going into a pocket universe that was outside of time and space.
Now my guess is that even though Power Girl is like a native to this earth-2 "2", it isn't the earth-2 "universe" that she was born in, so this JSA will likely be strangers to her, even though they'll act like they know her, but her JSA is on earth-1. Notice the differences in costumes? There's also the possibility that maybe the earth-2 "2" Robin may perhaps be the same age as Nightwing, or it might have been the art which was actually good.
To put it short, this isn't really the return of the pre-crisis DC multiverse, but the creation of the post-crisis DC multiverse which I hope becomes bigger then 52 universes to explain the elseworlds, even the possible upcoming ones, just so it doesn't become a problem in the future. Like say releasing the Phantom Zone energy inside the vortex caused a change that allowed the replication process to continue or something, while the corruption in the tapestry of events that Mister Mind caused to the alternate realities makes each new universe different which would be the elseworlds.
Thanks for the headache.
All they need to do is put back the original heroes Batman, Superman, Wonder Woman, Robin and the Huntress from Earth Two and fix up the weird void that was left in WWII from the original three and I would be happy.