Can a finite be eternal, if not then there's an eternal behind the finite. If not we would end up with a cycle of infinite regression. There is no doubt there is a God, even if people throw reason out of the window.
Kurama never said that, I still go by Naruro using KCM and losing RSM due to bad writing. However, in principle you're right it could be compared to 8G Guy's suicide attack, but without the physical power I suppose.
Madara was playing with Guy, he tanked an RSM lava rasenshuriken. Isshiki is way beneath Kaguya, let alone Madara. If he's slapping around Adult Naruto and Sasuke then they're weaker than their teen selves by a significant margin.
Maybe, but Jobbersuke has negligible stamina/chakra. He's always out of chakra, lol. It would make Sasuke look even more pathetic if he loses to somebody who got solod by Darui.