A target that is cowering is ALSO considered to be shaken and panicked. The effects are not cumulative, but the conditions themselves are.
That is not true. You're thinking of shaken/frightened/panicked. Cowering is a separate condition entirely, although it is often associated with them.
The entire text of Cowering, from the SRD:
"The character is frozen in fear and can take no actions. A cowering character takes a -2 penalty to Armor Class and loses her Dexterity bonus (if any). "
This is in contrast to Shaken, Frightened, and Panicked, all of which have text referring to the other two as increasingly or decreasingly severe states of fear. The panicked state will inflict cowering if the creature is unable to flee, but cowering does not automatically imply any of the three fear states, much as (for example) the dying condition causes a creature to become unconscious but being unconscious does not automatically imply that you are dying.
This is a common misconception and leads to a lot of confused assumptions about how these states interact.