It's possible to appreciate his accomplishments without blinding oneself to his flaws--and, conversely, condemn the flaws without denying the accomplishments.
This, so very much. See also: Alfred Hitchcock, H.P. Lovecraft, Abraham Lincoln, etc, etc,
If Buffy had been cancelled one episode before Angel was 'revealed' as a vampire (um... spoiler?) would that have been speculation? Or both the planned outcome of the choices they were making, and, you know, obvious?
Is it possible that they might have changed direction? Sure. But there has never been any indication that they were going to.
IMNSHO, yes, that would be speculation. I treat what is formally pubished as being canon. Any word-of-god stuff is not canon until it shows up in actual, published material. Doesn't matter if it was an episode away, a season away, or a spinoff series away. For a very related example, the plot of the Annointed One in Buffy changed at the absolute last minute. He was supposed to be the big bad of season two. It was planned all through season one, was written that way completely thought season one, and didn't change until they wanted to start filming and realized the child actor had aged too much.
I actually look down on creators who try and force things into their product through "word-of-god" decrees. For a specific example, J.K. Rowling angers me with stuff like Dumbledor's sexuality. You had 7 main books, an infinite number of tie-in opportunities like Beetle the Bard, and 10 movies. If he's gay, have the guts to show it somewhere in the darned series already. If the only place you actually say it is on Twitter, it doesn't count.