What's with that assumption? I'm definitely a mathematically inclined person, I just don't care if you pull any numbers out of your ass. It was obvious that you only made assumptions, how could you have a good enough sample size for a precise number? Furthermore who cares about the exact cut point? Even if you're below it, spending HE on it is worth it as long as you have enough to manage advents (and there are a lot of things factoring into if that's the case). I love theorycrafting, but pointless theorycrafting is just a waste of time.
Yes, maths don't lie, but your base assumptions are entirely wrong. You're assuming that the KH that would replace Nike contributes nothing at all to the team besides plain autohitting. From my point of view, a current dmm light team will always consist of: Michael, Metatron and most likely SSArty. That means Nike would be competing for the 4th slot against Take (way more damage than Nike would be doing outside of her assault buff + ~1m added to each burst), Tsuku (5% more def down, Nuke and blind) and maybe Sol if you really need heal/cleanse/dispel. And that's the thing, it's near impossible to just compare the damage/utility any of these adds to the assault Nike adds. It's definitely completely wrong to completely ignore it the way you did.
Lastly: For everything you assumed perfect scenario of endless fights. Is it possible that Nike outperforms the 3 mentioned above on long fights? Absolutely, though by far less than your calculations suggest. But that's an extremely rare situation. Almost nothing takes 12 turns to kill once you got a strong team for said element. Ea is by far the best solution for stun punisher fire situations. Those also don't really exist right now, but you wouldn't call her the "strongest hime in the game" just because she excells in one redundant thing, that would be ridiculous. If something is strong on paper, but has no use currently, it's still useless.