I don't think the dog food will harm the cat but it doesn't contain enough protein for cats who need an almost all protein diet. Dogs need less and it wouldn't be good for them to eat all meat cat food.
Is the cat eating dog food because you also have a dog? or do you just buy it for the cat?
Difficult to do much about it wully. I have both dog and cat and keep the cat food on a window sill where the dog can't get it. The dog much prefers the cat food!! Cats have a mind of their own. I can't see that your cat will come to much harm. If she was a kitten then the lack of protein might cause a problem, but not so much with an adult cat.
Try feeding her and the dog in different places and removing the dog food when the dog is not eating. Just deny her access and she will probably eat the cat food if she is really hungry.
Definitely not, cats need taurine and other ingredients that are not in dog food, and the levels of ash are different. You will end up with a malnourished cat needing vets treatment if you only feed it dog food.