Question about Blade Runner and Deckard.

So a little thought in the replicant theory.

So I'd he is a replicant then his memories are false. What I want to know is, if Deckard is a replicant then why did they make his past that he had left the force, that they had to arrest him to make him work? Why not make him for purpose?

So Rachael is new, so Deckard must be too. So he can't have a backstory before the memories because the whole memory implant was first put in Rachael.

So is Deckard the first and Rachael the second? If so why did the first be a Blade Runner?