That is an itneresting theory, but it can be proved wrong easily.

First, here is the Diamond's mural:

Notice how their gem is always on display. If White Diamond was a fusion, why does she only have one gem on a place (her forehead) none of the other diamonds have?

And something else to prove it wrong: Remember how Pink Diamond's symbol was removed after the war?

If White Diamond really was a fusion that could not be formed anymore, there would be no purpose in keeping her symbol, they would have removed it like Pink.

No, White Diamond is real…. Remember when Zycron said only a diamond could have planned Pink's shattering, yet we saw Yellow and Blue grieving genuinely for her?

That only leave one diamond suspicious…. The only diamond that not even bothered to attend the trial of her dear sister's murderer…

That background statue thing….. Pink Diamond? I thought they had tried to erase her image. Or is that White Diamond? Their hairstyle kinda look alike.

Speaking of White Diamond, we didn't even got a namedrop. Worst, no one acknowledged her absence, not Blue Diamond, not Yellow Diamond, not even the lawyer when she was making her case.

Something is wrong. Something is seriously wrong….