>>38478001First of all by the first book in ATLA the characters were much more fleshed out and complex than at the start.
Secondly this season was written with the intention of completely ending at "Endgame". What we watched for Episode 12 was intended by Bryke to be the series finale, and in their eyes 12 episodes was "perfect".
It was not Nickelodeon who wanted it to be 12. It was Bryke. Nick ordered more episodes, Byrke never requested or intended for the show to be more than 12 episodes.
Now that they've finished writing Season 2 they've expressed interest in a third season and Nick will obviously greenlight it, but these 12 episodes we saw were supposed to be the entire "Legend" of Korra, if Nick hadn't asked for more.
Stop being such an apologist and just accept that the show was horribly written.
Even Bryke has accepted this. They said that they wanted to write the series on their own as a sort of challenge, but they hired some ATLA writers for Season 2 so even THEY know how clueless they are when it comes to writing.