I can't help but notice you have made the thread with something of an agenda.
In the end it boils down to one thing.
Katara needs to be powered up by the full moon in order to blood bend. A Full Moon being the time when a water bender is at his/her strongest.
Amon doesn't.
Katara resisted the bloodbending of someone who, you guessed it, needed to be powered up by the full moon in order to use it.
Amon can already use it normally and is further boosted by the full moon here; so really, Katara's ability to resist Hama is all sorts of irrelevant.