[EbeneezerAl]

Theyt are some interesting theories, to be sure. But I think you're reading a bit too much into it. I'd put most of it down to coincidence. There are a lot of consitencies between the various FF stories. You could make the same argument of the lifestream for the Cycle of Souls in IX (though as FFIX was made as an homage to it's predecessors, that might actually be an intentional similarity).

There are more things about the stories that are significantly different. The geography is very different. Some of the key locations of FFX are absent in VII, namely the Farplane. No pyreflies are relased from dying enemies in FFVII.

And finally, two of your connections don't work. The Cetra can't simply be the inhabitants of Spira because they were bigger than that. There was more power and significance to the Cetra. And we know that that wasn't a myth either because of Ifalna and Aeris. The other is that of Jenova. In order to make the comparison here, Jenova would have to have been Sin, not Yunalesca. Jenova was a direct enemy of the Cetra, while Yunalesca was simply an obstacle, albeit a significant one.

So again, it's an interesting theory. And it's intirely possible that those were intended as homages to FFVII. But I don't think they're any more than that.