Yeah, its the only explanation that I could think of that would make it so an avatar was alive in the opening. It kind of makes sense in the lore, as neither the pure spirit (the avatar) or the corruption spirit (the dark avatar) can be destroyed for good, at least not while the other exists.
This is all more speculation: It could also make sense how the PC lived to 18 without any bending, as the dark avatar was weakened after the battle, almost destroyed. Water was the first element he learned, due to him previously being Unalaq, a water bender in his past life, as this seemed to be the easiest for the PC to learn. He then gained the ability to learn the other elements from possessing aang's body, as he adjusted to what the host could do. Next as fire was the element Aang learnt last the spirit sped up the process by possessing zuko or the thief both likely firebenders, this was needed as Aang learnt firebending from Zuko before the final battle. Next it's back to Aang, as he now has some earth bending knowledge, and so he can learn from Toph at the time the real Aang would have done. Finally you would inhabit an air bender, Korra? Meelo? Kai? Or another unnamed acolyte with bending.
I feel the dark avatar would be a water bender in every reincarnation as the first dark avatar only new withstanding, so the spirit, possibly even vaatu in disguise as the painted lady is gearing the PC up to be a real dark avatar with all four elements, as he almost defeated Korra, a fully realised avatar with withstanding and dark avatar power alone.
