I don't get the whole passing off theory as fact. If there's no story-related official in-game, Vana'diel tribune or otherwise mention from SE, it is not fact and not something to attempt to be corrected. You can't say you're all wrong, this is how it is. You can say this is the theory you believe and back it up with why you believe that is. I can recognize that Terrestial avatars being guardians of the mothercrystals is not fact. It is a sound theory.

FACT: Phoenix controls the natural passage of life and death to and from the mothercrystals including herself.
FACT: There are 5 Terrestial Avatars who have guarded Vana'diel at least since the time of the Zilart/Ku'luu, albeit in their own individual ways of how Vana'diel should be saved.
FACT: The Terrestial Avatars by proxy protect the mothercrystals because it will end life on Vana'diel.
FACT: The Terrestial Avatars are not gods, they are mortal beasts who can die.
FACT: Terrestial Avatars can be ressurected.
FACT: Terrestial Avatars can merge with a host and later be removed from them.
FACT: Terrestial Avatars chose to become what they are.
Quote Originally Posted by Diabolos, Waking Dreams quest
Still, I became a terrestrial avatar in order to assist mankind, and I fully intend to continue my work.
FACT: Carbuncle is not manipulating anyone. Carbuncle asks Summoners to help him on his quest. Manipulating would require him misleading summoners and it being proven in the story.

Theory: Terrestial Avatars have nothing to do with Altana and were created by happenstance.
The problem I have with the theory that the Terrestial Avatars have no role and simply commonplace animals back then is where are the rest of the overpowered avatar animals? Why are there only 5? Why are they the only ones concerned about saving Vana'diel? Why are the roles of the Terrestial Avatar's so instrumental in the grand design? Why did Altana even give them powers if all animals aren't created with that much power? It leaves too many unanswered questions to support that theory for me.

Theory: Carbuncle is evil or has partially evil intent
The problem I have with the theory that Carbuncle is in some way even partially evil, is there is nothing to support it. Just because he wants to awaken the sleeping gods does not make ill intent. Naivety maybe, but not evil. He states in Waking the Beast, he had no intention of the humanity's will being bent by the Celestial Avatars. He does think the world needs to be restored to it's true form (Source: The Moonlit Path quest). We have no idea what the true form of Vana'diel is. There is no proof that waking the celestial avatars is equivalent to opening the Gate of the Gods, ending Vana'diel. If that were true, then the Zilart would have just tried to awaken them and Carbuncle would have went to the Zilart to begin with and wouldn't have helped stop the Gate of the Gods to be opened.

What type of avatars are Cait Sith and Atomos?
They are simply avatars, like others have mentioned. Just because we have two sets of categories, does not mean there cannot be a 3rd or more. Hopefully the quest to unlock Cait Sith will tell us more but so far all we know is Cait Sith is the only avatar to actually be created directly from Altana (Source: WotG storyline). Where the celestial avatars use to be men and women (Source: Vana'diel Tribune) and the terrestial avatars use to be animals (Source: Carbunce: The Moonlit Path).