When we got our rescue retriever at a week short of nine, she was a physical wreck. The rescue shelter had done wonders for her, but she was still very ill.
She couldn't eat beef for quite a few months. She threw it up. Anything 'rich' was beyond her. We worked in chicken, and eggs, then fish, lamb and finally beef. Gradually she became what her ancestors were - a hardcore omnivore with a cast iron stomach. It took close to 3 years.
To argue with the concept of unclean foods is to argue with faith, I'm afraid. It's black and white, and never really a debate. It is or isn't to those who have this way of thinking.
Years of neglect in a kennel had made our dog an eater of poop, so her religion has a very different view of unclean than anyone here's. Now she's off the dog droppings 100%, but she does have a great love of the gifts deer leave around here. That's an omnivore for you.
She's made it to 14 and is doing well (knock on wood). In my tanks, I feed my omnivores, piscivores, detritivores, insectivores etc differently. I try to adapt to them, and not make them adapt to me. At the start of all this I said feeding a vegan or vegetarian diet to dogs would be not sensible. It can never make them vegans. I rarely eat meat, not because of scriptures but because I don't. If I were to take a few more steps and become a vegetarian 100%, I would still be an omnivore making a choice.