There is a difference between control and influence. Shadowbenders do not control someone directly, only get them to move in the direction they want, it's no different than convincing someone to do something they otherwise wouldn't do, only they are unaware there's another person involved, they think it's all their idea. Haven't you ever thought something and said to yourself "Where did THAT come from?!", same thing.

The whole trick of shadow benders is that really they are no powerful than any other nations, it is their strategies (stealth and manipulation of the other nations) and the their "alien" discipline (no one has seen shadowbending for thousands of years, and so it has been lost to time and memory) that make them seem so powerful.

Shadowbending is like the mugger sticking the finger in his coat pocket in your back, and it's too dark for you to know for sure if it's a gun or not, so you regard him as a threat. The problem is that he also has a knife in that pocket and has convinced your best friend you were going to betray him and so is helping the mugger, who is going to end up robbing both of you. It's the ultimate bluff, divide and conquer, and smoke and mirrors show, but there is also deadly intent, so it's still very dangerous.

1.) If the Dark Avatar hasn't returned to restore balance of +/- chi for a milenium, why has all been calm before the fire nation attacked 900+ years later?

The Dark Avatar Spirit??hasn't been imprisoned for 1000 years, 'millenia' can span many thousands. In this case, it would be very near the Beginning, which may not have necessarily been the first such Beginning. Remember what Roku said to Jeong Jeong (sp?) in 'The Deserter", "I have mastered the elements 1,000 times in 1,000 lifetimes. Now I must do it again. You will teach the Avatar firebending." This could mean that the seperation of??+/- Chi has existed for as long as 100,000 years, if not longer. As for the calm prior to 100 years. I see??a few??possibilities. One is that the orbit of Sozen's Comet was a spiral in an effort to restore the natural balance, and only recently has it been close enough to actually 'bend', thus tampering the seal placed on it and allowing the Dark Avatar to??influence the actions of the Fire Nation NOBILITY, which I differentiate from the common Fire Nation citizen. Second is that it was a stable orbit with a cycle of 100 years, but this creates problems in that if it had such a short cycle, why hadn't this kind of tampering been attempted earlier, the only reason being that the danger was common knowledge until it was lost to the ravages of time. Finally, the third possibilty could be that it was a long cycle, say 1000 years or more, and that when Sozen tampered with it, he altered its orbit into a shorter 100 year cycle.

2.) Why is the Avatar residing in Sozin's Comet, how did it get there, and it this a good thing for the fire nation or a good thing for the world?

The DARK Avatar Spirit was imprisoned in the Comet because it represented a threat to all, Spirit and mortal alike. The Spirits did not see any way to undo the damage that had been done, and the Dark Avatar was too dangerous to be permitted to reincarnate, ushering in cycles of chaos and destruction every generation. So, with the aid of the Spirits, even those more inclined to side with negative chi (such as the Ocean Spirit), The Dark Avatar was forcibly removed from the Avatar Cycle, and then sealed in a prison made up of a perfect balance of all four elements, Water and Earth for the core, Air and Fire for the head and tail, which with the skill of the eras greatest benders, was cast from the world into the limitless void. However, like lightning, the negative and positive wishes to restore balance, and come crashing together, so what we are seeing is the first part of a massive train wreck, so it's bad for everyone involved, but in the end, necessary for the world. It's all a matter of whether or not the Four Nations will be alive to see their world in balance again.

3.) What is the significance of the unique bending arts, how do they help Yin/Ying coexist peacefully?

Their significance is that they are forms of bending that should not exist. They are pure expressions of +/- Chi, and as such do as much harm to the user, save the Avatars, as they do to others. It's like trying to drink pure soda syrup; without diluting it, it'll kill you as it eats away at your insides with its??caustic nature. If Yin and Yang were to coexist peacefully as they should, they would dilute each other, making such arts impossible to perform. The only reason there are shadowbenders instead of lightbenders is that only the Avatar can give other benders the ability bend pure +/- Chi, and the Avatar of Light (which Aang is) would never teach such a dangerous and ultimately self-destructive art. The Dark Avatar has no such reservations, and would exploit the people foolish enough to seek shadowbending's seductive power to its own ends, which is to cause chaos and destruction, the desire to conquer and rule is given by the incarnation, or after it's release from the Comet, its host.

4.) Why are the Moon & Ocean spirits considered Yin, on the side of the Dark Avatar, what does this make the Sun spirit?

The Ocean is Yin, the Moon, Yang. The reason the Ocean would not side with the Dark Avatar is that the Ocean Spirit recognizes the need for Balance, and the Dark Avatar Spirit's only??desire is to upset that Balance. As for the Sun Spirit, very few Fire Nation nobility and military follow it, as they are unwittingly following the will of the Dark Avatar. So, for now, the Sun Spirit has been abandoned by the Fire Nation leadership essentially, save as a military resource. Also, I believe the Spirits??on the whole are in balance with their opposite patron/matron Spirit, and as such cannot show direct expressions of power. I noticed that the Ocean Spirit didn't show it's power until the Moon Spirit had died, upsetting the Balance and unleashing the Ocean's full power, which manifested itself by using Aang as a conduit of sorts. The return of the Moon Spirit caused this power to fade. This is how I believe the fight with the Dark Avatar Spirit will eventually end. I believe the Dark Avatar wishes to rejoin the balance by possessing Aang, but remain dominant over future reincarnations. Aang will eventually realize he must allow this to occur but not dominate or allow to be dominated, restoring true balance, and perhaps removing the need for an Avatar altogether.