I have a idea to keep her out of the littler pan. you could move the litter pan to a new location. this time to a room with a door.. and what ya do is buy one of those hook and chain security things,, making it so that the door only opens up a few inches. just enough for the cat to squeeze through the door and hopefully leaving the puppy out of trouble and letting the cat do its thing..

if puppy does manage to squeeze through the crack in the door,, you could try putting something in the way.. something the cat can jump over but not the puppy..

i hope that all makes sense.