CYK result de-transformation
I don't think you can recover a unit production, unless (and this is probably the common case) the unit production is unambiguous. If there is only one derivation from a non-terminal to a particular terminal -- and it should be easy to figure that out while you are doing the transformation to CNF -- then you can record that derivation and reinsert it when you find the collapsed derivation in the CYK parse.