Meh. It's more or less interpretive differences between all our theories (except for some of them). At least I could start somewhere for others to completely punish for saying anything wrong and saying something right.

If you can understand Japanese, the Ultimania guide should help for the deeper intricacies.

And for Vegnagun, it WAS developed to fight Sin, but never used because it could not differentiate between friend and nemesis; which is clearly stated in FFX-2, and I would not consider it a spoiler at all. (HAHAHAHA GET IT NEMESIS MUAHAHAH). Also, I can't Spoiler something with the eidt button, and I'm not typing out (SPOILER)XD