The recording is like any Pensieve recording- which shows the truth of the actions but not the motivations. Diggory could still claim that he had no idea Draco didn't mean to attack him, and Draco, if he were telling the truth, would have to admit that he deliberately provoked Diggory's attack for the sake of the recording spell. That wouldn't look all that great. And video, good as it is for many things, is far from being undisputed in court; look at something like the Rodney King case, which clearly showed police officers beating King on videotape, but didn't show their motivation for doing so (they claimed he was acting dangerous and they thought he was on drugs; others claimed that it was because of racism).

Diggory will, of course, have the ability to deny even if they present the recording at Harry's trial, but he won't have as much warning. He'll have to fabricate his own story on the fly. They're saving this for a surprise just as they're saving Littlesmith for a surprise witness, because they know long-term political plans will give the advantage to Diggory.

Kingsley hasn't been talking to much of anyone about his fears, so he lost perspective. Draco, Hermione, Harry, and Millicent are regularly in communication and can check each other.

I can't answer the question about Willowberry yet.

And thank you!