Well, the start of the series states that Korra doesn't have the spiritual side of bending down at all, so maybe she can't channel it even subconsciously? Or maybe she just hasn't been pushed far enough for it to activate. I've really got nothing as far as that, though I don't doubt that at some point she'll be pushed over the edge and it will activate in some way.
Ya, the Avatar shouldn't need any spiritual control to enter it in a purely defensive, uncontrolled mode. Like when Aang released the Avatar state during the storm as he fled the air temples out of fear of being seperated from Giazo.(sp?) Or when he found Giazo's bones at the air temple, or when the earth kingdom general threatened Katara's life, or when the sand benders stole Appa.
It really didn't take much to trigger Aang, he was a bit more of an emotional youngster. But given how much fear Amon has been putting into Korra, and how every time he touches her she gets flashes from Aang's memories, I'm really expecting that to be a rush of Aang's spirit into hers, like the beginnings of the Avatar state. Since that's pretty much what the Avatar state is, every spirit of the past Avatars inhabiting the current Avatar.
The unrealized release is there just to protect the Avatar before they learn to control it. Otherwise the Avatar would be a sitting duck.