For a start, the Nasrudin film was storyboarded in full --- there is a ton of photo evidence of this -- plus portions of the revised film to an extent -- and Williams was still using those old boards during the 1989ish production (there is video evidence of this) before doing the new boards at that point.
He was somewhat secretive about still using the old boards, but they were "good enough" before the movie went into full production.
Boards of the leads meeting the Brigands, and of the Grand Vizier's entrance into the city, for example.
A close-enough-to-final draft of the "Thief" script was in place by 1980 as well, about ten years before production began officially.
(And of course the Nasrudin film was fully scripted, and, again, storyboarded.)
Richard became somewhat secretive about the process of the film over the years, as he was worried about getting ripped off. This led to a lot of wild rumors about no storyboards existing and so on.
It is true that no major footage of the Cobbler or the Princess was completed until the 1990ish production began, and that the characters were redesigned around this point. Williams had avoided doing much storyboard work of these major characters up to this point, but storyboarded their scenes with the new designs then.
Pencil test video of the Brigands scenes during this production shows they were still using the Nasrudin-era boards when applicable.
Howard Blake is incorrect. Margaret French wrote the film and invented the character (in collaboration with Richard). Blake made an attempt at a treatment/concept that was not used in any way. This can be verified by reading what Blake wrote, which does not resemble the film, and by reading what French wrote, which does. (French claims Blake's version was never read by Dick, which I can't verify, but is plausible.) Blake wildly overstates his own involvement, since he's speaking from his own point of view.Howard Blake claims that he came up with the Tack character after Nasruddin fell through, but on the other hand some say that Richard and his wife invented him as early as the night he got the Oscar for A Christmas Carol, when they were considering the protagonist needed to be somewhat more appealing and sympathetic.