Maybe this belongs on the Fan Fiction site, or at least N&N, instead of the Books Forum which is usually reserved for academic arguments? I know a lot of Books threads deal with speculation, but if we're allowed to start posting fan fiction here, I will be one of the
first slugs out of that can of worms and you will regret it!
P.S. I should add that this scenario has already been looked at, at least in brief, in another fan fic. In this version the explanation was merely that the idea of the Three fading once the One was gone was merely another one of Saruman's lies (as in the Council the idea of the Three fading is actually one of two competing theories). This obviously doesn't achieve what you're trying to here, but it is relevant and shows that maybe (especially if you're going AU anyway) you wouldn't need to expend herculean Epic Effort. It can be found
here.