Ah, but then we have another point entirely, which is that the water plot in Fallout 3 is completely strange, unecessarily convoluted and inconsistent with the portrayal of radiation in the rest of the series. PossibleCabbage explained it throughly in the post I linked:
Well that is just completely untrue. NPCs in Fallout 1 and 2 mention glowing water, radioactive rain, and the need for Geiger counters, since radiation is still everywhere. The Midwest is described in Fallout 2 as a giant radioactive dust bowl.
Even in New Vegas, which made a massive deal about how House deflected most the nukes, keeping the area clean, many towns, such a Nipton, have their water supply, which comes from the local water tower, contaminated with radiation, it even has a small radioactive pool forming below the nozzle they get water out of. Even in pristine areas such as the Mojave, radioactive water is still a problem to many.
As for the quoted material.
A. All arguments using real world science are fallacious at their core, as Fallout is based on 50's pop-culture and scifi b-movie portrayals of radiation. Hence why radiation causes animals to mutate into giant versions of themselves, and how it makes humans into ghouls.
B. People in the D.C. area do filter water, they just lack the machinery to do it on such a large scale as to make farming viable.
C. You can't filter the water with dirt in Fallout 3, since the dirt is radioactive as well.
D. Radiation IS magic, hence the point made in A. That was always the entire point since the beginning. I do believe Tim Cain even equated radiation in Fallout to be like magic, and used that as justification as for why radiation can cause mutant animals as portrayed in 1950's scifi b-movies, and in the game.
E. The whole point about the GECK is a non issue, as they didn't use the GECK itself in Fallout 3. Both James and Braun state the GECK is dangerous, and doesn't really work like its supposed too. And James directly says he is going to use components of the GECK's technology to fix the purifier, not the GECK itself.
In short, everything you just posted is nothing short of pure historical revisionism about the nature of the world of Fallout, and things stated in Fallout 1, 2, and 3, layered on top of fallacious arguments of real world science in a game setting where realism never applied to the bomb.