Yeah... we can only speculate how Asa was captured.
As for what Enya said.... she didn't say all elemental shifters needed a bare minimum of their element to survive, she said that she did. Her element needs O2. So does her physiology. Humans need to breathe. We don't need light for survival. I'm sure water shifters would need at least a few sips to survive as well, but humans can survive in a dry environment. Earth shifters would have to be deprived of any earthy materials like dirt or rock, which humans can survive without.
We know that if the battle takes place in Siberia, Huntington will have a huge advantage and vice versa if it happens in Montana.
Who knows... maybe both sides will get lured to a totally different place like the remote jungle in South America.
I think Caleb is finally realizing he's been forgiven and is well loved.