I can confirm that those are the only premier skins in the grid. Kind of sucks that the ultimate edition ones aren't exclusive and cost 6k instead of 4k. I'm sure they will release more as most of the cast don't have any.Only The Flash having 2 is weird.
I beat the story mode too and it said something about beating all scenarios for an alternate ending. Possible something could be behind that. Didn't get any other unlocks besides brainiac.
Can you see the skins (locked) in the shader menu for each character from the beginning?
And by 6k, you mean the standard currency/credits you get from playing the game in general?