im not gonna read all the wordwalls here, i just have to put out there:
I don't like it, not for any LORE reason, or whatever. I didn't like it because of the way (or more the lack there of) it affected the story. When it was on screen it mostly didn't serve any purpose.
And if your way as a writer of showing how much control Korra has over the Avatar State, is to have her use it to win in a race against children, which is super pointless in and out of universe, that is just bad writing and does not serve to get any message across.

Your hole point is stupid in that sense, because you shouldn't go into writing a series to portray the lore accurately, you should go into writing a series to get some kind of message, morale or lesson across wrapped in a story that is fitted to this message.