Welcome, Ryurien!
So, first thing's first: Tidus's head
did get blown off, but not because he shouted "blitzball!" like an idiot and kicked it (because how the hell does one get their head blown off
kicking a bomb? I'm like, seriously?! Shouldn't his body starting from the feet and legs, be obliterated instead?). A bomb that was disguised as a blitzball was 'mysteriously' thrown in his direction, catching his attention. He then proceeded to walk over to pick it up and inspect it,
since why would Tidus have any reason to suspect it's a leftover weapon from a thousand years ago, and then BOOM! His head flies off. If you read the novel in its entirety, you'll get the convoluted gist.
As for the plot device that is Deus Sex Machina (
http://tvtropes.org/pmwiki/pmwiki.php/Main/DeusSexMachina), I can't really tell you for sure 100% its legitimacy. Since I have yet to reach those chapters in my Fan Restoration Project (feel free to read that side-by-side with the original chapters) to analyze it thoroughly, I believe that the Bevellian Summoners' method of producing a "living Fayth" through an Aeon Core is a nod to an ancient spiritual Eastern practice called "Tantric sex," which is thought to expand consciousness and join together the polarities of masculine and feminine energy into a whole.
A link to the aforementioned subject:
http://www.mindbodygreen.com/0-23332/tantric-sex-101-what-it-is-how-to-do-it.htmlI could talk more about that and its examples within the novel, but I want you to reach the chapters first before I spoil it for you.
I hope this cleared up some questions? If you have more, you are more than welcome to ask!