To stop a cat from playing with its food, get a spray bottle and fill it up with water. When the cat plays with its food, gently spray the water at the cat and then take the food away. Eventually the cat will learn that playing with the food triggers the unwelcome consequences.
This really depends on the quality of the food the cat is fed on, also how much food the cat eats in a day (the lower quality the food, the more the cat will eat). For a cat fed on a premium-quality wet cat food, it would probably eat approximately 15lbs of food per month.
I have a cat myself, but he has never ate dogfood in his life. Even so, cat food is made for cats, and dog food is made for dogs so it would be rare for a cat to eat dog food. But if your cat does eat it, then it probably won't do anything to your cat.