For different offenses, formations and teams, different names will be used to differentiate between different positions and roles within these more generalised names given to the positions.
The quiz doesn't need to account for all the variations because not all teams follow the same naming conventions. The quiz is good as it is.
I can't believe I missed linebacker, because my son was a huge baby and I call him my future linebacker!
Linebackers are separated by Inside and Outside; they are two very different positions. I would have ILB and OLB as different positions.
Nickelback isn't a position. A team will play a 3rd Corner Back when they are in a Nickel Defense, and that player is sometimes referred to as the Nickel Corner or Nickel Back, but nobody ever refers to themselves as the Nickel Back.
And Flanker is indeed a position. It is generally a wide receiver that does not line up on the line of scrimmage (as opposed to a Split End that does). The terms Split End (SE) and Flanker (FL) are not often used anymore, but if you are going to have one you should have the other.