I think what Ridley had in mind was sort of a film noir feel for BR, and a Sam Spade type of character for Harrison. Thus the original voice over, which he thought was unneeded.
The movie works better without the narration by Deckard (imo). The visual ambiance and the Vangelis soundtrack are perfect, and enough.

Hard to say about casting choices. I never felt Fords portrail of the Deckard character was flat. On the other hand, Gaff was played by Olmos perfectly. He's meant to be threatening. Where as Deckard is deep in the moral and ethical dilemma of his job.

Just a film aficionado. That's all. My brain is pretty much movie quotes, directions, and UOF tactics. Drives my wife nuts.