I still haven't watched my Final Cut yet. Good thing I saw that message before selling my Director's Cut, although considering FC is the genuine director's cut, one might say it is still the definitive Blade Runner cut, since it is what is intended.
As for Mercerism, it's funny you would mention that bit, because I was falling from any ability to believe in my religion as I read DADoES, and Dick effectively softened the blow one no doubt feels when they lose faith and wonder what of it was real or worthwhile. Now that's chicken soup for the soul.
That said, it's the kind of thing that is lost in translation from books to film. A good book creates and develops a world. A good movie shows you as much of that world as it can without becoming overly long. Mercerism would have hurt the film in the length department, and while books are encouraged to tackle more than one huge aspect of society and humanity, films typically suffer when they don't pick one and stick by it. Mercerism alone is enough for an entire film.
As for what a poor Harrison Ford fan Blackadder is, I believe it was said once that Ford wasn't a very big fan of the character he was playing, a "detective that didn't detect". While I couldn't disagree more, one might say that he channeled this inertia and uncertainty into his portrayal of the character, someone doing something he doesn't want to do, and questioning it every step of the way, to growing extents.
The more you know.