in people’s minds when Lee Adama asked “What if it had been Zak in the viper” after Starbuck returned.

However, your rule (known as the gun on the mantelpiece rule) does not apply here. Zak exists in the story for several reasons. First of all, he was a short-lived character in the original series, so the writers don’t have to have a use for him in mind when they put him there. Secondly, he has been used, as the thing that comes between Starbuck and Adama, since Starbuck’s mistakes got him killed.

Finally, “Adama is a Cylon” is now explained by Tamara Adama, Bill’s older sister. whose mind was a template for one of the first Cylons.