Again your missing the "memories" part.
Yes we understand that Deckard’s memories (of his ex-wife/job/life etcetera) works both ways in that they “could” be implants (which is a strong theme in PKD stories), however it is just too implausible to swallow considering just how “human” he behaved and how intricate these memories must be.
So he was activated... did they drop him off across the street from his favourite sushi bar (which of course is an implant as well) and sat him upright on the bench looking like a mannequin before flipping his on switch? (This is me being flippant here.

)
However to be a little more serious for a moment-
If they implanted him with the memories of a cop to go after these elusive/dangerous Replicants, they could have at least done a better job with his abilities. I’m not talking about superior strength, I’m talking about just being a more efficient cop.
He goes to Leon’s apartment to gather clues. Why not just stakeout the place and wait for Leon to show up (which Leon almost did if Deckard wasn’t there first). Was he programmed to make mistakes? Shouldn’t catching these Replicants take precedent over making Deckard appear as human (flawed) as possible (to himself as well as others)?
He is nearly killed in each encounter with the Replicants and survives through luck. Zhora is spooked by witnesses (when she could have easily choked him to death), his girlfriend saves him from Leon, Pris (who is horrible at fighting?) would have stomped his head in if it wasn’t for her wanting to showboat.
And of course Batty saves his life. This is how they programmed him? To get his butt kicked? He isn’t even a good shot!
His only success is shooting two unarmed women (one of which in the back).
I don’t get that if he is a Replicant, and they go to all the trouble of making him believe he is human, why they would also give him all the frailties of the human mind. Why send an inferior robot to deal with dangerous superior robots?
Of course there is also the lack of drama if he is a Replicant. If he is killed, they could simply re-implant the false memories into another model and send him back out as if nothing happened. As a viewer how are you supposed to feel any sense of impending peril if our hero is essentially “immortal”?
And yes; (just to say it) you are not going to change my opinion nor do I believe I will change yours. No worries there. :thumbsup However my hope is that I might sway someone else who might be on the fence about this stuff.
Kevin