Thread is long, I'm saving my opinion on the multiplier but I need to say this. 75% Shigaraki is vastly weaker than Complete Shigaraki.
Complete Shigaraki can tank attacks that can tear through the bodies of High-Ends, who have equal durability to 75% Shigaraki, with zero damage.
The limits of Complete Shigaraki have no defined value. 75% or Incomplete Shigaraki can't scale to or above the missiles, I agree with that, but Complete Shigaraki isn't bound by that same logic. Him and Prime All Might can be stronger or weaker than them depending on what we find.
This is also ignoring the fact that due to the surface area of the explosion and the inverse square law, Shigaraki wouldn't even be hit by 1% of the explosion. That is the real reason there is no scaling, not because of what you've said. It's already been agreed on that it's physically impossible for all of that energy to hit him.
We've calculated that the energy that would hit Shigaraki from the explosion is around than 1.9 Gigatons of TNT, which means 1.9 GT would kill him?
No, we're just ignoring the missile feat all together.