Secret, that is a delicious theory and would seem to make sense if it weren't for the unavoidable coincidence that the real Zenaida FG visited Sawgrass Apartments on June 17th, right when KC knew she needed to start building an excuse. And correct me if I'm wrong, but ZFG looked at the exact same apartment that KC said she brought Caylee for babysitting.
KC knew what ZFG did that day. She either watched her or got her name from the visitor's card.
I surmise that KC was obsessed with being a nanny, and yes KC is Zani the nanny. But she had been following/tracking this particular ZFG for some time, for some reason. When the time came for KC to do her dirty deed, she used ZFG as her scapegoat.
Her description of the real ZFG is someone that KC aspires to... she's a nanny with a lot of money, not tied down, Hispanic.
I hope and pray that LE has already figured out how KC got ZFG's name from Sawgrass.