I've suspected it probably wasn't permanent for awhile now, and Lin getting her bending taken away confirms that for me. Whether it's a complex chi-block, a mind trick, or something super Avatar hax is needed to fix, everyone's bending will likely return.
As for who Amon is, I'm thinking it's a manifestation of past Avatars trying to protect the world from having a future tyrant like Fire Lord Ozai. That would explain why Korra has been unable to communicate spiritually with past Avatars except for like a few Aang flashbacks.
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"THIS is how you make Nintendo win another generation. Good job, Sony and Microsoft, you're idiots." - Vlado