Can Zombify target a creature card that isn't in the graveyard?
I think that the reading of 115.2 you mentioned above is just too narrow.  Reposting it again:


  115.2. Only permanents are legal targets for spells and abilities, unless a spell or ability (a) specifies that it can target an object
  in another zone or a player, or (b) targets an object that can’t exist
  on the battlefield, such as a spell or ability. See also rule 115.4.


115.2 specifies that only permanents are legal targets for spells or abilities unless another zone is specified (or implied by the object type).  This implies one of two things is true:


It can only target creature cards that are currently permanents on the battlefield, and the card doesn't actually function at all.
"target [object] from [zone]" as a wording template does include a zone as part of the targeting requirements.


Because of how 115.2 is worded, there is not another option where it can target a creature card anywhere.  Either it can only target permanents, or its zone is specified (or implied by the type of object as in (b)).  Since the card does currently function, we have to assume that this is considered adequate language for specifying a zone as a part of targeting requirements.