Yes, Ea works by dissolving the Scroll of Creation. However, that is just one more example of a way to get higher than expected power out of a limited mass-energy budget. And 'dissolving the scroll of Creation' is not a very difficult trick for Odyssial to figure out. Also, Ea is indestructible, so we know the Grail can make at least weakly indestructible objects.
It doesn't have to be a raw power thing, that's the point of efficiency - if it has a particular configuration of mass-energy that can generate effective invulnerability, then we know effective invulnerability is within the mass-energy budget. Odyssial doesn't work by raw power either, he works on skill, which is typically very cheap to incorporate from an energy standpoint - consider the skill difference between a nuclear physicist and a human-sized deer, yet they require the same amount of energy.
Humanity includes Zelretch. Un-nerfed Zelretch can destroy the world with infinite energy fairly easily. Same with Aoko.
It doesn't matter what the way specifically is; if the way exists, he will find it and exploit it. Attaining all previously acquired magics is therefore the minimum of what he would do, if it is possible at all. It's also very likely that he would reach Akasha and become effectively omnipotent over the Nasuverse, sure. And of course, he can master every field of 'normal' thaumaturgy with greater skill than the highest mages of Babylon, much of which is 'on the level of Magic' as well. If there is any power that a human or humanlike mind has ever attained by any means - artifice, skill, etc - Odyssial will near-instantly acquire it and refine it to, at minimum, the maximum possible level.
Perfected Eyes of Death Perception. Perfected alchemical calculation on a level surpassing Dust of Osiris. Perfected combat technique on a level orders of magnitude greater than Tsubame Gaeshi, but for every single move he makes. That one is particularly promising since it allows reality warping through sheer skill alone, and skill costs almost nothing in terms of mass-energy to manifest, it's just a specific configuration of existing mass. Perfected wide-area prana absorption on a level vastly surpassing Night on the Blood Lair. And so on and so forth, all of them synergistically amplifying each other.
For example, we know that the Knight Arm, Slash Emperor, is capable of killing TYPES including Gaia (by drawing energy from the world). Given the already-stated feats of beings such as Zelretch, Aoko, Ryogi Shiki, etc, who are far less capable than Odyssial, this is highly problematic for the survival of the setting.
This is all assuming he doesn't find some way to regain his original powers (99+% chance this happens), which turns it into an instant stomp without discussion. I don't think one should put someone that can fight the Downstreamers into the Nasuverse, they don't share a meaningful reference class. It's like a modern supercomputer versus the ENIAC - they cost about the same amount of mass-energy to make, it's the configuration that makes one trillions of times more powerful than the other.
The Nasuverse is simply not strong enough to contain someone of Odyssial's level, even if he has to be spawned in nerfed form from the Grail. There are just too many ways to get around that, whether from efficiency due to pure skill, or one of the countless ways to get near-infinite prana that a human or humanlike mind has already demonstrated. Remember, he is more dangerous than a Xeelee self-replicator spawned from the same mass-energy budget.
