spiral energy
is about doing the impossible, but the more impossible feats in the show require way more spiral energy - Simon has to spend minutes on end generating spiral energy at points towards the end of the show to make shit happen. Violating fiat
is something i think spiral could theoretically do, but i imagine it would take spiral energy levels far beyond what was managed in even the final fight of the show where they were throwing around entire galaxies like it was nothing. And in the process of getting to that point, per LordRoustabout's statements about spiral, you would necessarily exceed the level of spiral energy that would trigger the gurren lagann endjump scenario wherein the anti-spiral shows up to either kick your ass or force you to kick all of spiral-empowered humanity's ass.
if we assume that fiat is stronger than the anti-spiral, which i will presume that it necessarily must be in order to make something like the endjump scenario trigger, then the baseline level of spiral needed to trump fiat is 'stronger than the strongest spiral warrior ever canonically seen' - defeating the anti-spiral is only theoretically impossible due to overbearing firepower, but violating fiat is
conceptually impossible; it's not something that just has a zero-percent-chance of succeeding, it's something that is impossible to even begin to attempt. Spiral is essentially about your willpower exceeding your limitations; fiat happens to essentially be the strongest limitation that one can conceive of, so it's gonna take a shit ton of spiral to defeat it. like, beyond anything you'll ever be capable of generating beyond the author of your story deciding that you have the most exceptional will-power that can be conceived of.
it also depends on what exactly the fiat effect is, and whether spiral could simply be used to supplant it; if you have a power that says "you cannot use this power to do [x]" and you want to defeat this limitation using spiral energy... why not just use spiral energy to do [x], instead? on the subject of the duplicates sustaining their existence or something... wouldn't it be simpler to use something like the thingy that preserves their memories across duplicates alongside spiral to preserve the rest of them in some way?
No, it has more to do with 'will'. They are completely fine with their finite existence. If they intended to be their own persons with all their being, Spiral would have attempted to fill the gap.
this, sorta - the duplicates being unbothered by joe's baggage is a symptom of their limited existence, so to try to use spiral to extend their limited existence, they'd be taking that baggage unto themselves and therefore become less capable of using spiral energy on par with joe prime.
