I took my time, I admit it...
The system seems fairly interesting, but I have a few doubts.
So, once again, I'm not really "fond" of having a "combat related" ability based on a secondary ability, the reason being that you can easily reach 440 style by a very low level, something you CAN'T do with primary abilities. Just consider that at Level20 I rolled characters with 400 Attack and Defense (and I guess you might even push them a bit more), and at the same level I rolled characters with Occult 1000 or Science 1240. You can easily see how growth of secondary abilities isn't even comparable to that of primary abilities.
My proposal is basing Bending on MK+d100 capped by the Base Style value. MK grows rather reasonably (it depends on class, of course), and Style would still be important, since it determined the highest level the ability can reach.
As a general rule, I believe the MK cost of most bending abilities is a bit low. I would use 20MK for those you price 10MK and 30MK for those you price 20MK. As for the rest, I'm a bit disappointed with Bending abilities stopping at Level 6. I would rather stretch the powers more along levels (or add a few powers), reaching a Level14 final ability.
Also, it's unclear how you maintain powers, I'd make it through "Innate Bending Slots" working like those for Sustained Psychic Powers, paying each slot 20MK.
For a single element 1CP should be enough, I believe.
3CPs and +1Level Modifier would be more appropriate than 4CPs on my opinion, since it looks like something you're born with, hence a Blood Bond. I guess energy bending removes bending ability forever...Now, what happens when the ONLY AVATAR loses the Pow Contest and is the one losing Bending? There's no more avatar in the world? I guess this makes sense in the setting, perhaps, but for game purpose I believe that having its effects last for a certain time, based on a roll as for other bendings, would be more fun.
WATER WHIP: First two levels should cause Fatigue Loss. Starting from fourth difficulty level, I'd allow each difficulty level of Bending to increase the Impact Strength up to 15 at Zen level.
ICE BARRIER: More HP needed here on my opinion.
ICE SPIKE: Minimum difficulty should be Very Difficult (or else Fatigue Loss as per Ice Barrier). I'd make it 1XSpike each two difficulty level (starting with the fifth), so 1 Spike at difficulty Very Difficult/Absurd, 2 Spikes at Almost Impossible/Impossible, 3 Spikes at Inhuman/Zen.
BLOOD BENDING: PhR100 is definitely low for a "final ability". This time base difficulty should be absurd (and lower difficulties should be fatigue loss). I'd set PhR as following: Absurd120, Almost Impossible140, Impossible160, Inhuman180, Zen200.
STONE LEVITATION: You're allowing an attack with base damage 3840...it's sick, really. I'd rather say Damage = 1/10 of weight in LBS rounding down at multiples of 5.
ROCK WALL: A bit more HP and an increasing Damage Barrier (which could be doubled by Metal Bending).
EARTH SPIKE: Start with Very Difficult and add additional spikes as for Ice Spike.
ROCK ARMOR: Start with Very Difficult and AT2. Then have it increase by 2 each difficulty level up to a maximum of 12. Shouldn't affect Energy.
EARTHQUAKE: Star with Very Difficult with 50ft radius...than procede as normal.
METALBENDING: Should raise by 1 Difficulty Level the minimum Difficulty to use Control Earth, Stone Levitation, Rock Wall, and Rock Armor and add 2 Fatigues in case of failure.
EXTINGUISH FIRE: PhR240 at Zen. It's more than enough.
FIRE MISSILE: Start with Difficult, and add +10 Damage per additional difficulty level up to 110 at Zen.
IGNEOUS BARRIER: Instead of an attack from the Bender, I'd go for a Predetermined Attack Value equal to difficulty level achieved.
JET PROPULSION: I'd go for base difficulty Very Difficulty with Flight 8 and leave the rest untouched.
LIGHTNING BENDING: Ok, although in Gaia electricity is usually associated with Air and not Fire. Remove Electric Acr, instead grant Fire Missile the option of using Electricity instead of Fire and add +50 to the Base Damage (so Base Damage ranges between 100 and 160). This makes so that Fire Benders have exactly the same number of powers as other benders. I like simmetry.
WHEEE!: Halve Movement bonus beyond Movement 10 (rounding up, of course). Start with Easy +1/+10 and get up to +9/+90 at Zen.
PUSH: Start with Difficult at STR8 and increase by 2 each additional difficulty level up to a STR20 at Zen.
AIR CUT: Start with Very Difficult and have opponent make a Notice Check against the difficulty level achieved or search check against one difficulty level lower or suffer from blindness.
AIR BUBBLE Start with Very Difficult and leave the rest untouched.
WHIRLWIND: Start with Difficulty Level Absurd. Have Predetermined Attack equal to the Difficulty Level achieved. Grant +2STR to the check each additional Level (so that Zen grants STR20).
These are just suggestions of course. The system itself seems pretty interesting on my opinion and looks like a very good job.