First of all, I never saw that disclaimer until the early 1990s, like 93 or 94. Welles died in the mid-1980s, at the height of Turner's colorization. So the causation seems a bit off.

So we would have a ROTJ directed by Spielberg, Gary Oldman as Grievious...

Don't forget that Lucas also considered David Lynch. As for Gary Oldman, blame his overzealous agent. According to Matthew Wood himself, Oldman only submitted a voice audition, along with several other actors. He did it as a personal favor to Rick McCallum, who is a friend of his. He was never selected. According to other sources, his agent announced and then retracted his casting. Needing a cover for his gaffe, he came up with the SAG story.