Cetra
Ok. Having compared the English script with a fan translation of the Japanese, I think I have it.
The Cetra are, as all versions agree, an itinerant people. The translation,      however, says that they move in, "unlock a world" and move on      again, rather than "settle a world" as the English script would      have it. (Let us leave aside, for the moment, mistranslation issues; the      actual point here works equally well whether you read this line as "world"      or "land".)
Put this together with what Sephiroth says about humans using the "gifts" of      the Cetra without working for them or repaying them in the proper Cetra      way.
What is the single greedy-human goal, throughout this game? Mako. Fertility      for the land and energy for the cities. Which is to say, the Lifestream.
This seems to imply that what the Cetra do is wake places up--allow the life      and total consciousness of the planet to manifest as the Lifestream.
Considering the awakening or consciousness-raising aspect of this, and the      manner in which humans are castigated for laying aside this calling and      this journey, I'm really seeing the Cetra as a sort of amalgam of the high      points of Buddhist and Shinto philosophy. The basic animism of the living      and aware planet is classic Shinto. The journey strikes me as equally classic      Buddhist. Recall that the hallmarks of Cetra are to be aware of the planet's      life and to be able to communicate reliably with it, yet never to stop,      never to settle, never to become attached, and to trust that this journey      will lead them to total peace and happiness in the end. Sounds like Nirvana      to me.
Thus, it makes perfect sense that, when a group of Cetra abandoned the journey      and the detachment demanded by it, they lost both their perception of the      living planet and their ability to be one with it. Which, of course, is      how the Nibelheim notes suggest humans came to be.
The Cetra being the midwives, if you will, of the Lifestream, and the Lifestream      being the manifestation of both spiritual oneness and environmental equilibrium,      the Cetra are strongly identified with harmony--stability, of the sort that      homeostasis is. Over against this we have Jenova.
Jenova
We are given two main aspects of Jenova. One is pure destructiveness. Jenova      seeks to eradicate the Cetra and, once part of the Sephiroth-Jenova meld,      seeks to destroy the planet and use the planet to destroy others so that      Sephiroth can plunder the energy of the Lifestream(s). A more subtle aspect      is transformation. Sephiroth tells Cloud that the nature of Jenova is transformation,      and we see this played out in both the SOLDIERs, in Hojo's reactor experiments,      and in the Jenova-bits that Sephiroth uses as carriers for his appearances.
The translation of Cloud's reflections upon regaining his memories tells us      that the process of creating SOLDIERs and Copies is the same--Jenova cells      plus Mako exposure. In the first, however, the will of the human is strong      enough to suppress the random transformation that, in unsuccessful cases,      produces monsters. Indeed, Sephiroth's will is strong enough to completely      reverse the flow and transform Jenova-bits into his own likeness. In the       case of the Copies, the will of the human is weak and psychic erasure results,      leaving the humans to eventually (presumably) merge into a new Jenova upon      reunion.
What all this suggests to me is that Jenova is the avatar of destabilization,      in this world. This makes a nice opposing contrast with the Cetra's harmony,      and is reflected in the opposition of the Black and White materia.
Heros
The lever that determines which will triumph, though, is neither Cetra nor      Jenova. It's the humans. Aerith, half human and half Cetra, and Cloud, a      human who has assimilated the Jenova transformation and imposed his integrity      of self on it, are the two halves that make up the hero in this story.
The same is true of the villain, of course. Sephiroth is very much altered,      but he was born of two humans and his alterations are due to an exposure      process very similar to the one Cloud seems to have undergone at Hojo's hands. It just happened to Sephiroth a lot earlier.
Thus, the conflict and resolution, here, are not between the Cetra and Jenova.      They already had their fight, and it's over. The Cetra are dead and Jenova      sealed. The conflict now may use the tools of Cetra (the Black and White      materia) and of Jenova (her transformative cells) but it is waged by humans      and, in the end, it is decided on their level.
It is not, you note, Holy that counters Meteor. It is, rather, the Lifestream      itself. And the last image we see, as that battle is fully joined, is Aerith      in the green-light and rising sparks of the Lifestream. This implies some      involvement on her part, and I don't think it's unreasonable to assume that      she was the one who instigated the Lifestream to rise; being half Cetra      she could hear it and communicate, both, and, as we find out in later installments,      also maintain her individual integrity within the Lifestream. Holy, like      the Weapons, eradicates any threat to the planet, up to and possibly including      humans themselves. The Lifestream, on the other hand, encompasses humans;      all life, including humans, rises from and returns to the Lifestream. And,      in the end, the Lifestream rises to protect Midgar, a concentration of no      other life but humans.
This is actually why I rather think Aerith was the motivator for that salvation.
It's Sephiroth, who despite his wish to be a god hasn't gotten there yet,      who calls Meteor and holds back Holy with the force of his will. It's Aerith      who calls Holy and, perhaps, the Lifestream as well, by the force of her      will. It's Cloud who must gather himself to defeat Sephiroth's will with      the force of his own. Their limited, attached, human wills are what drive      this whole story.
Humans
That will not only reaches beyond the things of the Cetra, but also beyond      Jenova. As Cloud tells Tifa, it's strength of will that determines whether      a human given SOLDIER or Copy treatments will succumb to Jenova or maintain      physical and mental integrity. Sephiroth is consistently, alarmingly, successful      in overcoming Jenova's will and even bending her transformations to his      own ends.
So is Cloud.
He cannot, at first, maintain mental integrity, hence the amnesia and identity      blurring and imposition of Sephiroth's will. But he maintains physical integrity      from the first. It's possible, of course, that Sephiroth was responsible      for that, wanting an undercover puppet, as it were. But, given the strength      of will Cloud displays as the game goes on, the simplest explanation is      that Jenova never could overcome Cloud. And, once he got his mental ducks      in a row, neither could Sephiroth. In fact, ironically enough, since Cloud      eventually defeats Sephiroth, displaying equal and greater strength of body      and will, Cloud may be considered the only truly successful "copy"--the      only one who walks the same path of transformation that Sephiroth walked      and a little bit further.
Indeed, it is suggested in Advent Children that it's Cloud's very attachment      to the human lives and embeddedness in the world around him, his humanity      and not his many-times-removed Cetra heritage or his Jenova exposure, that      make him stronger than Sephiroth.
What I find most interesting is that, in the game, this is not sufficient      for victory.
Cloud's journey and victory are very personal and do not actually have any      direct affect on saving the world. That's made very clear to us as he and      his friends watch helplessly as Meteor threatens to destroy Midgar and maybe      the world, despite their individual victory. It's Aerith who takes the world-savior's      part. She's the one who summons Holy and, I think, the Lifestream to deal      with the world-destruction Sephiroth started in motion. It's necessary for      Cloud to get Sephiroth and his will out of her way, but it's equally necessary      for Aerith to take the macro-level steps to counter that will.
Despite the obvious fact that only Cloud has grown strong enough to fight      Sephiroth directly, it is made subtly, blindingly clear that saving the      world is a group effort. No one person is enough. And, as Sephiroth's defeat      demonstrates, no person alone can hope to win. It's a classic collectivist      moral, which is a darn good trick in a fighting RPG.